Optical ring networks having node-to-node optical communication channels for carrying data traffic

ABSTRACT

Techniques, apparatus and systems for optical communications, including fiber ring networks with protection switching to maintain optical communications when an optical failure occurs and to automatically revert to normal operation when the optical failure is corrected, fiber ring networks that provide a circulating optical probe signal at an optical probe wavelength within the gain spectral range of optical amplifiers used in a fiber ring network to detect an optical failure, and fiber ring networks that support broadcast-and-select optical WDM signals carrying communication traffic to the optical ring nodes without regeneration at each optical ring node and one or more overlaid in-band node-to-node optical signals carrying communication traffic with regeneration at each node.

PRIORITY CLAIM AND RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/175,422, filed on Jul. 17, 2008, and entitled “Optical Ring NetworksHaving Node-To-Node Optical Communication Channels For Carrying DataTraffic” which claims the benefit of U.S. Provisional Patent ApplicationNo. 60/950,327 entitled “Optical Ring Networks Having Node-To-NodeOptical Communication Channels For Carrying Data Traffic” and filed Jul.17, 2007, the entire disclosures of which are incorporated by referenceas part of the specification of this patent application.

BACKGROUND

This application relates to optical communication networks.

Optical ring networks use one or more optical ring paths to opticallylink optical communication nodes. Each optical ring path may be formedby fibers or other optical links. Such optical ring networks can includea single fiber ring in some implementations and two separate fiber ringsin other implementations. Either uni-directional or bi-directionaloptical communication traffic can be provided in optical ring networks.Optical ring networks can have various applications, including theaccess part of a network or the backbone of a network such asinterconnecting central offices. Optical ring networks can beimplemented to provide a protection switch as a “self-healing” mechanismto maintain continuous operation when an optical break occurs in theoptical ring and can also allow for relative ease in adding and deletingnodes on the optical ring. In addition, WDM optical rings can providedirect peer-to-peer connections through wavelength add/drops withoutexpensive regenerators. Furthermore, the cost of optical fiberdeployment in a ring topology is generally much less than that in a meshtopology. Due to these and other features of optical ring networks,various optical ring networks have been widely deployed in metro andregional local area networks (LANs) for both data communication systemssuch as a token-ring LAN and Fiber Distributed Data Interface (FDDI)LAN) and telecom systems such as SONET/SDH optical networks.

Like other optical networks, an optical ring network may experience anunexpected break point in the signal traffic. For example, a fiber maybreak by, e.g., a fiber cut or a failure of an optical component in thering such as an optical amplifier. The ring topology of optical ringnetworks allows a protection switching mechanism to be implemented formaintaining the operation of the optical ring network in presence of thebreak and for restoring the normal operation after the break isrepaired.

The current carrier-grade quality of service requires the protectionswitching time to be less than 50 msec. Different protection switchingmechanisms can be implemented in optical ring networks to meet thisrequirement. For example, SONET rings and resilient-packet-rings (RPRs)have been introduced to support efficient packet switching while meetingcarrier-grade quality of service requirements mostly via SONET physicallayer interface, including the protection switching time less than 50msec. SONET or RPR rings currently deployed usually useoptical-electrical-optical (O-E-O) regenerators to connect nodes to thering and thus the O-E-O conversion is present at every span. This use ofthe O-E-O, conversion can limit the overall capacity of the network tothe capacity of the span with the smallest bandwidth in the ring.Therefore, when capacity upgrade is needed in SONET or RPR rings, everyspan of the ring network needs to be upgraded and such upgrade isreferred to as fork lifting update and can be costly. Examples of thefork-lifting upgrade include: (a) a 2.5 G SONET ring grows to a 10 GSONET ring by upgrading every SONET ADMs in all nodes, and (b) a GigabitEthernet ring grows to a 10 Gigabit Ethernet ring by upgrading everyswitch/routers in all nodes.

Alternatively, WDM or DWDM optical ring networks can be implemented withall optical add/drop nodes on the ring without expensive O-E-Oregenerator so that nodes are connected directly by multiple DWDMwavelengths to offer much higher capacity, reduced timing jitter, andimproved signal latency and to allow for scalability, all at a reducedcost. Such an all-optical DWDM ring network relies on optical layerprotection, whose recovery time is normally well within the currentlyrequired protection switching time of 50 ms for the carrier-gradequality of service.

SUMMARY

This application disclose techniques, apparatus and systems for opticalcommunications, including fiber ring networks with protection switchingto maintain optical communications when an optical failure occurs and toautomatically revert to normal operation when the optical failure iscorrected, fiber ring networks that provide a circulating optical probesignal at an optical probe wavelength within the gain spectral range ofoptical amplifiers used in a fiber ring network to detect an opticalfailure, and fiber ring networks that support broadcast-and-selectoptical WDM signals carrying communication traffic to the optical ringnodes without regeneration at each optical ring node and one or moreoverlaid in-band node-to-node optical signals carrying communicationtraffic with regeneration at each node. An optical node in such a ringnetwork can include a reconfigurable optical add and drop multiplexer(ROADM) for reconfigurable add and drop operations of optical WDMchannels.

In one aspect, this application describes an optical communicationsystem that includes optical ring nodes connected to form an opticalring which support (1) broadcast-and-select optical WDM signals carryingcommunication traffic to the optical ring nodes without regeneration ateach optical ring node, and (2) at least one node-to-node optical WDMsignal carrying communication traffic from one optical ring node to anadjacent optical ring node by regeneration at each optical ring node.The node-to-node optical WDM signal is at a wavelength different fromwavelengths of the broadcast-and-select optical WDM signals. This systemalso includes optical amplifiers coupled in the optical ring andoperable to amplify light in a gain spectral range covering opticalwavelengths of the broadcast-and-select optical WDM signals. Thenode-to-node optical WDM signal may be within the gain spectral range ofthe optical amplifiers in some implementations while outside the gainspectral range of the optical amplifiers in other implementations.

In another aspect, this application describes an optical communicationsystem that includes optical ring nodes connected to form an opticalring having a first optical ring path to carry light through the opticalring nodes along a first direction and a second optical ring path tocarry light through the optical ring nodes along a second, oppositedirection. The optical ring is configured to support (1)broadcast-and-select optical WDM signals carrying communication trafficto the optical ring nodes without regeneration at each optical ring nodein each of the first and second optical ring paths, and (2) at least onenode-to-node optical WDM signal carrying communication traffic from oneoptical ring node to an adjacent optical ring node by regeneration ateach optical ring node in each of the first and second optical ringpaths. Each ring node includes: a broadcast-and-select add and dropmodule to produce at least one of the broadcast-and-select optical WDMsignals and to split a fraction of light in the broadcast-and-selectoptical WDM signals for detection while transmitting rest of the lightin the broadcast-and-select optical WDM signals; a node-to-node opticaltransmitter to produce the node-to-node optical WDM signal; an opticalcoupler to receive the node-to-node optical WDM signal from thenode-to-node optical transmitter and to split the node-to-node opticalWDM signal into a first node-to-node optical WDM signal and a secondnode-to-node optical WDM signal; a first optical add coupler coupled tothe first optical ring path to direct the first node-to-node optical WDMsignal to the first optical ring path; a second optical add couplercoupled to the second optical ring path to direct the secondnode-to-node optical WDM signal to the second optical ring path; a firstoptical drop coupler coupled to the first optical ring path upstreamfrom the first optical add coupler to selectively couple light of thefirst node-to-node optical WDM signal out of the first optical ring pathwhile transmitting light of the broadcast-and-select optical WDMsignals; a second optical drop coupler coupled to the second opticalring path upstream from the second optical add coupler to selectivelycouple light of the second node-to-node optical WDM signal out of thesecond optical ring path while transmitting light of thebroadcast-and-select optical WDM signals; and an optical receivercoupled to receive the first and second node-to-node optical WDM signalsfrom the first and second optical drop couplers.

In another aspect, this application describes a method for opticalcommunication in a ring network. This method includes usingbroadcast-and-select optical WDM signals at different wavelengths tocarry communication traffic to optical ring nodes in the ring network,where each optical ring node splits a fraction of thebroadcast-and-select optical WDM signals for detection whiletransmitting rest of light of the broadcast-and-select optical WDMsignals to a next optical ring node. At least one node-to-node opticalWDM signal is used to carry node-to-node communication traffic from oneoptical ring node to an adjacent optical ring node by regeneration ateach optical ring node. The node-to-node optical WDM signal is at awavelength different from wavelengths of the broadcast-and-selectoptical WDM signals. In this method, each broadcast-and-select opticalWDM signal is designated to a selected optical ring node for sendingdata from the selected optical ring node to one or more other opticalring nodes; and the optical ring nodes are operated to share a bandwidthof the node-to-node optical WDM signal to transmit data from one opticalring node to one or more other optical ring nodes through thenode-to-node communication traffic.

In another aspect, this application describes an optical communicationsystem which include a central node and multiple optical ring nodesconnected to form an optical ring. The optical ring nodes support (1)broadcast-and-select optical WDM signals carrying communication trafficto the optical ring nodes without regeneration at each optical ringnode, and (2) at least one node-to-node optical WDM signal carryingcommunication traffic from one optical ring node to an adjacent opticalring node by regeneration at each optical ring node. The node-to-nodeoptical WDM signal is at a wavelength different from wavelengths of thebroadcast-and-select optical WDM signals. The central node is connectedto the optical ring and structured to comprise anoptical-to-electrical-to-optical (OEO) block which converts receivedbroadcast-and-select optical WDM signals into electrical signals and toregenerate the broadcast-and-select optical WDM signals that travel tothe optical ring nodes. The central node is structured to support theone node-to-node optical WDM signal carrying communication traffic froman adjacent optical ring node to another adjacent optical ring node byregeneration of the one node-to-node optical WDM signal.

This application also describes optical communication ring networkshaving all optical add/drop ring nodes and a protection switchingmechanism using a circulating optical probe signal in each fiber ringfor detecting an optical break point in the optical rings and foroperating the protection switching mechanism. The optical wavelength ofthe circulating optical probe signal can be a designated wavelength thatis different from optical wavelengths of optical WDM signals carryingWDM signal channels and is, like the optical wavelengths of optical WDMsignals, within the operating gain spectral range of optical amplifiersin the optical ring networks.

In one aspect, this application describes an optical communicationmethod in an optical ring network carrying optical WDM signals andincluding optical amplifiers operable to amplify light in a gainspectral range covering optical wavelengths of the optical WDM signals.In one implementation, an optical probe signal is coupled to the opticalring network and is at a probe wavelength which is inside or near oneend of the gain spectral range to obtain a sufficient optical gain fromthe optical amplifiers to sustain a detectable signal level. The opticalprobe signal is monitored in at least one optical node in the opticalring network to detect an optical failure in the optical ring network. Aprotection switch mechanism in the optical ring network is controlled,in response to a status of the monitored optical probe signal, tosustain communications in the optical ring network outside a location ofthe optical, failure and to automatically restore communications in theoptical ring network after the optical failure is repaired.

In another aspect, this application describes an optical communicationmethod that can be implemented as follows. Optical ring nodes areconnected to form an optical ring network carrying optical WDM signalsin first and second, opposite directions. Each ring node includes atleast one optical amplifier operable to amplify light in a gain spectralrange covering at least optical wavelengths of optical WDM signals inthe optical ring network. If certain node-to-node interspan is short andno optical amplifier gain is needed, the optical amplifier is replacedby a conventional 1×1 optical switch. Each ring node is operable totransmit a first portion of light in the gain spectral range, includingthe optical WDM signals, and to drop a second portion of the light inthe gain spectral range. A first optical probe signal is coupled to theoptical ring network to circulate in the optical ring network in thefirst direction and a second optical probe signal is also coupled to theoptical ring network to circulate in the second direction. Each of thefirst and second optical probe signals is at a probe wavelength withinthe gain spectral range of each optical amplifier in each ring node. Thefirst and the second optical probe signals are monitored to detect afailure that causes a break in the optical ring network.

In the above method for the optical ring network with first and secondoptical probe signals, a central protection switch mechanism in aselected ring node can be used and controlled to create a default breakpoint for each circulating optical signal in the optical ring networkwhen there is no break point, other than the default break point, in theoptical ring network for each circulating optical signal and to closethe default break point when there is a break point in the optical ringnetwork for each circulating optical signal. The selected ring node maybe implemented in the following two different configurations.

In a first configuration, the selected ring node is used and operated asa central node to control the central protection switch mechanism andoperations of other ring nodes. A node-to-node communication mechanism,such as an optical supervision channel (OSC) signal at an OSC wavelengthoutside the gain spectral range of each optical amplifier in each ringnode, can be used to provide optical communication between twoneighboring ring nodes. The optical communication between twoneighboring ring nodes via the OSC signal is used to providecommunications between the central node and each ring node for operatingthe central protection switch mechanism and for operating each ringnode. Within each ring node, a portion of each of the first and thesecond optical probe signals is dropped from the ring network whileallowing a remainder of each of the first and the second optical probesignals to continue on in the optical ring network and the droppedportion is monitored to detect whether the first and the second opticalprobe signals are present. The central node is operated to receiveinformation from other ring nodes and to control other ring nodes inresponse to the received information.

In a second configuration for the selected ring node, the first and thesecond optical probe signals are coupled to the optical ring networkwithin the selected ring node. The selected ring node is operated todrop a portion of each of the first and the second optical probe signalswhile allowing a remainder of each of the first and the second opticalprobe signals to continue on in the optical ring network. The droppedportion is used to monitor presence or absence of the first and thesecond optical probe signals to decide whether there is a break pointelsewhere in the optical ring network. When at least one of the firstand the second optical probe signals is absent in the selected ringnode, the selected ring node is operated to control the centralprotection switch mechanism to close the default break point, withoutrelying on information from other ring nodes. When both of the first andthe second optical probe signals are present in the selected ring node,the selected ring node is operated to control the central protectionswitch mechanism to open the default break point.

Also in the second configuration, a ring node, that is not the selectedring node, monitors a total amount of optical power in each of the firstand the second directions in the optical ring network, withoutseparately monitoring each of the first and the second optical probesignals alone. Without relying on a command from the selected ring node,the ring node is operated to create a local break point in each of thefirst and the second directions within the ring node when the totalamount of optical power in each of the first and the second directionsis detected to be below a shut-off threshold, thus leading to shuttingdown optical transmission of a neighboring ring node.

In yet another aspect, this application describes various opticalcommunication systems. In one implementation, a system includes opticalring nodes connected to form an optical ring network which carriesoptical WDM signals. the optical ring network includes opticalamplifiers operable to amplify light in a gain spectral range coveringoptical wavelengths of the optical WDM signals. The system also includesan optical probe transmitter coupled to the optical ring network tosupply to the optical ring network an optical probe signal at a Probewavelength which is inside or near one end of the gain spectral range toobtain a sufficient optical gain from the optical amplifiers to sustaina detectable signal level. A probe monitor in at least one optical ringnode is included in this system to split a portion of the optical probesignal and to monitor the optical probe signal to detect an opticalfailure in the optical ring network. A protection switch is provided inthe optical ring network to create a default optical break point whenthere is no optical failure in the optical ring network and to close thedefault optical break point when there is an optical failure.Furthermore, the optical ring network in this system is responsive to astatus of the monitored optical probe signal to control the protectionswitch and the optical ring nodes to sustain communications in theoptical ring network outside a location of the optical failure and toautomatically restore communications in the optical ring network afterthe optical failure is repaired.

In yet another implementation, an optical communication system isdescribed to include optical ring nodes connected to form an opticalring network which carries optical WDM signals; and a protection switchin the optical ring network to create a default optical break point whenthere is no optical failure in the optical ring network and to close thedefault optical break point when there is an optical failure. In thissystem, the protection switch includes a first optical terminal toreceive an input signal, a second optical terminal to output an outputsignal, a first optical path comprising first and second opticalswitches connected in series, and a second optical path comprising thirdand fourth optical switches connected in series. The first opticalterminal is coupled to join a first end of the first optical path and afirst end of the second optical path to split the input signal betweenthe first and the second optical paths, and the second optical terminalto join a second end of the first optical path and a second end of thesecond optical path to combine light from the first and the secondoptical paths to produce the output signal. This switch can be operatedto provide redundancy in switching operations.

These and other implementations, examples and variations are nowdescribed in greater detail in the drawings, the detailed descriptionand the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one example of selection of the optical probe wavelength at1563 nm or 1564 nm beyond the usual spectral range from 1530 nm to 1562nm in the C band for WDM signals.

FIG. 2 shows an exemplary dual-fiber ring network having all opticalring nodes in a broadcast-and-select architecture that implements thecirculating optical probe signal for protection switching in acentralized control mode.

FIG. 2A shows an example of a wavelength selective coupler forselectively dropping a portion of the circulating optical probe signalin each node, where a fiber tap coupler is used to drop a portion oflight at all wavelengths and a probe bandpass filter to select onlylight at the probe wavelength for detection by an optical detector PD1as the probe detector.

FIGS. 3A, 3B, 3C and 3D show an example operation sequence of the ringnetwork in FIG. 2 when both fiber rings are cut at a location.

FIGS. 4A, 4B and 4C show an example operation sequence of the ringnetwork in

FIG. 2 when one fiber ring is cut at a location.

FIG. 5 shows an exemplary dual-fiber ring network having all opticalring nodes in a broadcast-and-select architecture that implements thecirculating optical probe signal for protection switching in a hybridcontrol mode.

FIGS. 6A, 6B, 6C and 6D show an example operation sequence in a hybridmode of the ring network in FIG. 5 when both fiber rings are cut at alocation.

FIGS. 7 and 8 show operations of a special non-central node next to theside of the central node with gate switches in the hybrid control modein the ring network shown in FIG. 5.

FIG. 9 shows one implementation of a node having a wavelength blocker toallow for reuse of a dropped WDM wavelength.

FIGS. 10A and 10B show two exemplary redundancy designs for opticalswitching devices.

FIGS. 11A and 11B show two operation modes of an optical switchingdevice based on the designs in FIGS. 10A and 10B.

FIG. 12 shows logical communication relations provided by dedicatedbroadcast-and-select optical WDM signals in a ring network.

FIG. 13 shows logical communication relations provided by dedicatedbroadcast-and-select optical WDM signals and overlaid in-bandnode-to-node optical WDM signals in a ring network.

FIG. 14 shows one example of a dual-fiber ring network that implementsboth dedicated broadcast-and-select optical WDM signals and overlaidin-band node-to-node optical WDM signals for carrying optical traffic.

FIGS. 15 and 16 show two exemplary implementations of optical nodes thatprovide generation and detection of a node-to-node optical WDM signal ata selected WDM wavelength.

FIG. 17 shows an example of a ring network that has regular opticalnodes and a central node equipped with multi-channel DWDM multiplexers,multi-channel DWDM demultiplexers, and a centraloptical-to-electrical-to-optical conversion block, where both dedicatedbroadcast-and-select optical WDM signals and overlaid in-bandnode-to-node optical WDM signals are used for carrying optical traffic.

FIG. 18 shows an example of a reconfigurable optical add and dropmultiplexer (ROADM) for reconfigurable add and drop operations ofoptical WDM channels in one direction in a ring network that implementseither one or both of (1) the circulating optical probe signal in thering and (2) the node-to-node communications.

DETAILED DESCRIPTION

Optical ring networks, e.g., in a single fiber ring configuration or adual-fiber ring configuration, can be used to support opticalcommunication traffic in two opposite directions. Hence, a particularsignal channel can be carried by two counter-propagating optical signalsin a ring network to provide redundancy. When the optical signal in onedirection is blocked or fails to reach a particular ring node, thecounter-propagating optical signal carrying the same signal channel canbe directed to reach the same ring node via a different route. This is asimple and effective way to improve the reliability of optical ringnetworks. Optical protection switching in optical ring networks has beendesigned and deployed based on this simple concept.

Optical protection switching in a ring network described in thisapplication can be implemented to provide, when a failure occurs in thenetwork, a protection mechanism for providing continuous communicationtraffic amongst the ring nodes that are not at the location of thefailure. After the failure is corrected, the optical protectionswitching can restore or revert the ring network back to its normalstate and operation. The reversion back to the normal state can beautomatic. In addition, during the normal operation of a ring when thereis no optical failure, the optical protection switching can maintain asingle default optical break point in a ring or each ring of a networkwith two or more interconnected rings to prevent formation of a closedoptical loop in each ring which can lead to re-circulating of light andthus undesired laser oscillation due to the presence of opticalamplifiers in the ring.

In actual implementations, certain designs of the optical ring nodes,the control mechanism, and the protection switching based on the controlmechanism can vary significantly in hardware complexity and cost, insystem operation and performance, and in system maintenance andreliability. As an example, in metro or regional optical networks,reconfigurable optical add/drop modules or multiplexers (ROADMs) canenable any-to-any (or meshed) traffic pattern on an optical ring networkdue to the flexibility in dropping/adding any wavelength at any locationin ROADM-based ring networks. However, if the optical fiber protectionscheme is not designed properly, the impact to the cost and thetransmission performance can be significant and may even compromise theflexibility of the ROADMs in ring networks. Some two-fiber opticalprotection schemes for ROADM-based ring networks provide the protectionswitch per wavelength. Such designs may not be economically feasible inpart because ROADMs are usually applied to an optical ring network witha large number of optical WDM wavelengths, and protection switchingbased on an individual channel can be costly as the number of opticalWDM wavelengths increases. Various other protection schemes are limitedin certain aspects, e.g., long transmission distances and delays such asin optical Bidirectional Line Switched Ring (BLSR) protection scheme inthe protection switching, penalty in the optical signal-to-noise ratio,and unacceptable interruption of services when the protection switchingis activated.

Designs and communication methods for optical communication ringnetworks described in this application use all optical add/drop ringnodes and a protection switching mechanism to provide reliablecommunications. A circulating optical probe signal is provided in eachdirection of the ring network to monitor and detect an optical breakpoint in the ring network and the detected status of the circulatingoptical probe signal is used to operate the protection switchingmechanism. The circulating optical probe signal travels through all ringnodes in the ring network and is common to and shared by all ring nodes.The optical probe wavelength of the circulating optical probe signal canbe a designated wavelength that is different from optical wavelengths ofoptical WDM signals carrying WDM channels and is, like the opticalwavelengths of optical WDM signals, within the operating gain spectralrange of optical amplifiers in the optical ring networks. This selectionof the optical probe wavelength allows the optical probe signal toco-propagate in the same optical path in the ring with the optical WDMsignals and to share certain network hardware, e.g., in-line opticalamplifiers, with optical WDM signals in the ring. This sharing ofhardware can minimize hardware specially deployed for the optical probesignal and thus simplify the network structure and reduce the cost ofthe network and the maintenance. The optical probe signal is monitoredin at least one optical node for detecting an optical failure in theoptical ring network.

In implementations, the optical probe wavelength can be selected at awavelength within the operating gain spectral range of opticalamplifiers to achieve certain advantages. For example, the optical probewavelength can be at or near one end of the gain spectral range with asufficient optical gain from the optical amplifiers to sustain adetectable signal level, i.e., shorter than the shortest optical WDMwavelength that is used by or reserved for an optical WDM signal, orlonger than the longest optical WDM wavelength that is used by orreserved for an optical WDM signal. Depending on the width of theoptical gain spectral range of the optical amplifiers, the optical probewavelength may be selected at the marginal region of one end of theoptical gain spectral range with an optical gain less than the gainnormally required for a WDM signal channel but still sufficient tosustain the circulating optical signal in the ring at the detectablelevel. This selection of the optical probe wavelength ensures that theoptical probe signal does not occupy any wavelength for the optical WDMchannels and leaves the central region of the gain spectral range of theamplifiers for use by WDM signal channels. The optical probe signal is“in band” with the optical WDM signals because they all generally fallin the gain spectral range of the optical amplifiers used in the ringnetworks. In other implementations, the optical probe wavelength may beat a wavelength in the central region of the operating gain spectralrange of optical amplifiers.

FIG. 1 shows one example of the optical probe wavelength at 1563 nm or1564 nm beyond the usual spectral range from 1530 nm to 1562 nm in the Cband for WDM signals. This selection of the optical probe wavelength maybe subject to the roll-off at each end of the optical gain spectralrange of the optical amplifiers, e.g., a few dB less than the opticalgain for the WDM optical signals located within the central region ofthe optical gain spectral range. As long as the optical gain for theoptical probe signal is sufficient to maintain the optical probe signalstrength at a detectable level throughout the ring, the probe wavelengthfor the optical probe signal can be as far away from the WDM signals aspossible to preserve as many wavelengths for use for WDM signals. Inmany WDM ring networks, an optical supervision channel (OSC) is used fornode-to-node data communications for the network management and controland is launched at one node and dropped at an adjacent node. The OSC isusually at an “out of band” wavelength outside the spectral range of theWDM wavelengths, e.g., at 1510 nm while WDM signals are around 1550 nm.Such an out-of-band OSC wavelength is not amplified by in-line opticalamplifiers for amplifying the WDM signals. Due to this lack of signalamplification and due to the presence of optical loss in the ring, itcan be difficult to circulate an OSC signal around the ring andtherefore an OSC signal is usually used between two adjacent nodes andnot shared by all nodes. At each end of a node, an OSC transceiver isprovided to include an OSC transmitter to generate an outgoing OSCsignal to the next adjacent node and an OSC receiver to receive anincoming OSC signal from the next adjacent node. Hence, informationcarried by the OSC is passed on from one node to the next by using anOSC receiver to convert a received OSC signal into an electrical signaland then using the an OSC transmitter to generate a new OSC signal tothe next node.

The circulating optical probe signal in each direction of a ring networkdescribed in this application is very different from the OSC signals inpart because it circulates through all nodes in the ring network withoutbeing re-generated at each node, and is shared and used by all nodes.Optical amplifiers stalled in the ring network for amplifying thein-band WDM signals also amplify the optical probe signal to sustain theoptical power of the probe signal at a sufficient level. The circulatingoptical probe signal can be used to provide an effective and efficientway of monitoring and communicating the status of the entire ring. Insome implementations, such a circulating optical probe signal can beused exclusively for detecting and communicating an optical failure suchas a fiber cut or a device failure in the ring network without carryingother network management and control information and thus a separatenode-to-node communication mechanism such as the OSC signaling can beused to communicate other network management and control information todifferent nodes in the ring network. In other implementations, thecirculating optical probe signal can be modulated to carry other networkmanagement and control information and to communicate such informationto different nodes in the ring network. Hence, a separate node-to-nodecommunication mechanism such as the OSC signaling may be eliminated.

Optical ring networks that implement the circulating optical probesignal for protection switching can have various ring configurations.Both single-fiber rings and dual-fiber rings may be used to supportoptical traffic in two opposite directions for redundancy. The ringnodes in such ring networks can vary based on specific requirements ofthe network applications. In some implementations, the ring nodes can bedesigned to include a central node and regular nodes. The central nodecan include either an optical switch or an optical amplifier which isshut down in each direction so as to maintain a break point in eachdirection in the entire ring network. The in-band circulating opticalprobe signal can be launched and detected at the central node andcirculates around the ring except for the default break point in thecentral node to check and monitor the continuity of the entire ring. Ifthere is any break due to a fiber cut or a component failure in opticalfibers, optical amplifiers, or even jumper cables, the propagation ofthe circulating optical probe is interrupted by the break and can beimmediately detected at the central node due to loss of the circulatingoptical probe in the direction along which the central node isdownstream from the location of the break.

FIG. 2 shows an exemplary dual-fiber ring network 200 having all opticalring nodes in a broadcast-and-select architecture that implements thecirculating optical probe signal for protection switching. This ringnetwork 200 includes optical ring nodes, e.g., nodes 210, 220 and 230,connected in two fiber rings 201 and 202. The fiber ring 202 is aclockwise (CW) fiber ring where each optical signal circulates in theclockwise direction and the fiber ring 201 is a counter clockwise (CCW)fiber ring where each optical signal circulates in the counter clockwisedirection.

The ring nodes in the ring network 200 include a central node 210 andregular ring nodes 220 and 230. As an example, each ring node can beimplemented with two amplifier line cards each having an opticalamplifier for signal amplification in one of the two counter-propagatingdirections. The central node 210 in this example has a first amplifierline card 210A as the interface on one side of the node 210 that has anoptical amplifier 212 to amplify WDM signals and the circulating opticalprobe in the clockwise direction, and a second amplifier line card 210Bas the interface on the other side of the node 210 that has an opticalamplifier 216 to amplify WDM signals and the circulating optical probein the counter clockwise direction. The amplifier line card 210A addsone or more add WDM signals in the counter clockwise direction and dropsWDM signals in the clockwise direction for the node 210. The amplifierline card 210B adds one or more add WDM signals in the clockwisedirection and drops WDM signals in the clockwise direction for the node210. Hence, light in the clockwise direction in the network 200 entersthe central node 210 via the amplifier line card 210A and leaves thenode 210 via the amplifier line card 210B. Regular ring nodes 220 and230 are similarly constructed with amplifier line cards 220A and 220B,230A and 230B, respectively.

The regular ring nodes 220 and 230 can be configured to share the samenode structure with identical node components. The central node 210 hasa different node construction from the regular ring nodes 220 and 230and includes central gate switches 211 and 212 in the two fiber rings201 and 202, respectively, and an optical probe transmitter 203 thatgenerates the circulating optical probe signal. In addition, the centralnode 210 has a network control mechanism that analyzes data from regularring nodes 220 and 230 and sends out node control commands to theregular ring nodes 220 and 230 for execution of certain node operations.Under this design, each regular ring node detects the circulatingoptical probe signal and other signals and reports the detected data tothe central node 210. The central node 210 processes detected dataobtained within its own node and received detected data from regularring nodes 220 and 230, and in response, controls certain actions in theregular ring nodes 220 and 230 to carry out the protection switching. Assuch, the design in the ring network 200 is a centralized control designand all decisions and actions for protection switching are controlled bythe central node 210.

In this centralized control mode, the network 200 uses node-to-nodecommunication signals for exchanging data and messages between thecentral node 210 and the regular ring nodes 220 and 230. Suchnode-to-node communication signals are present in the ring network 200along with the circulating optical probe signal and the optical WDMsignals. Various node-to-node communication techniques may beimplemented. The network 200 in FIG. 2 shows an example of anode-to-node communication mechanism based on optical supervisionchannels (OSCs) that use out of band optical wavelengths to provideoptical communication between two neighboring nodes for managing andoperating the protection switching mechanism according to predeterminedcontrol algorithms for maintaining a single break point (or a singlebreak span) in each ring path. Other node-to-node communicationmechanisms different the OSC signaling may also be used to carry othernetwork management and control information to different nodes in thering network.

As illustrated, each node includes an OSC module 218 with OSCtransmitters and receivers for generating and receiving OSC signals inthe two separate fibers 201 and 202. Because each node has twoneighboring nodes, the OSC module 218 includes two OSC transceivers 218Aand 218B for communicating with the two neighboring nodes, respectively.Referring specifically to the central node 210 and the ring node 220,the OSC transceiver 218A in the central node 210 has an OSC transmitter(T) to send an OSC signal in the counter clockwise direction in thefiber ring 201 via the line card 210A to the OSC receiver (R) in the OSCtransceiver 218B of the ring node 220 via the line card 220B. The OSCreceiver (R) in the OSC transceiver 218A of the central node 210receives the OSC signal in the clockwise direction in the fiber ring 202via the line card 210A from the OSC transmitter (T) in the OSCtransceiver 218B of the ring node 220. Wavelength-selective OSC couplers(not shown) such as optical filters can be used to add and drop the OSCsignals while allowing the optical probe signal and the optical WDMsignals to pass through. Therefore, the OSC signaling between twoneighboring nodes allows for the node-to-node communication. For OSCsignaling between two non-neighboring nodes, an intermediate node canrelay the OSC data from its OSC transceiver 218A on one side to its OSCtransceiver 218B on the other side.

Each ring node, including both the central node 210 and a regular ringnode (e.g., 220 and 230), has an optical WDM add/drop module to generateoptical add signals 206A and respective replicas 206B to the twodifferent fiber rings 201 and 202 in the two opposite directions and toreceive optical drop signals 207A and 207B from the two different fiberrings 201 and 202. Optical broadband couplers can be used to provide theoptical add and drop couplings while allowing the optical probe signaland the optical WDM signals to continue to propagate after passing eachnode and after a small portion of all the WDM signals and thecirculating optical probe signal is split off and dropped to the node.The ring network 200 in FIG. 2 is a broadcast-and-select network in thecontext that each node can broadcast an optical WDM signal to all nodesand select one or more desired channels from the dropped signal whichcontains all optical WDM channels in the network to receive. One or morefixed or tunable optical filters may be used to select one or moredesired WDM channels for each node from the optical drop signals 207Aand 207B. A WDM demultiplexer may also be used to separate the droppedWDM signals in the drop signals 207A or 207B and then the one or moredesired WDM channels are detected and processed to extract data.

Various designs for the add/drop module may be used in FIG. 2. In theexample in FIG. 2, the add/drop module has first and second add/dropmodules 205A and 205B that are respectively coupled to two line cards ontwo sides of each node, respectively. The module 205A has a transmitterto add one or more WDM signals to the fiber ring 201 in the counterclockwise direction while dropping all WDM channels from the fiber ring202 in the clockwise direction. The module 205B has a transmitter to addone or more WDM signals to the fiber ring 202 in the clockwise directionwhile dropping all WDM channels from the fiber ring 201 in the counterclockwise direction. The signal add and drop functions at each node canbe implemented by using a broadband coupler such as a fiber tap coupleron the ring to drop or add optical signals and an off-ring opticalsplitter coupled with one or more optical filters or an opticaldemultiplexer to separate dropped signals, and an off-ring 1×N combineror multiplexer to combine different add signals together for coupling tothe ring via broadband coupler on the ring. Different optical addsignals produced from different optical transmitters in each node can becombined by a wavelength multiplexer or a 1×N combiner. Both an opticaltransmitters operating at a fixed laser wavelength and a tunable opticaltransmitter producing a tunable laser wavelength may be used to producean add optical signal.

The central node 210 is designed to place the signal add coupler and thesignal drop coupler on the same side of the gate switches 211 and 212for the protection switching. Two optical couplers for coupling twooptical probe signals for the two opposite directions in the ringnetwork are respectively placed on two opposite sides of the gateswitches 211 and 212 so that the opening and closing of the gateswitches 211 and 212 do not affect coupling the optical probe signals tothe ring network.

Each regular ring node 220 or 230 in FIG. 2 has one optical amplifier217 in each fiber to amplify light in one of the two counter-propagatingdirections so there are two amplifiers 217 in each regular node 220 or230. These two amplifiers 217 in each regular ring node can be locatedin two separate amplifier line cards. Under a normal operatingcondition, each optical amplifier 217 in a regular ring node 220 or 230is activated and turned on to amplify light. When there is a breakoutside the central node 210 in the ring network 200, as part of theprotection switching, at least one optical amplifier 217 in one or tworegular ring nodes adjacent to the break point is turned off and becomesdeactivated to function as an open optical switch to create a protectivebreak point. In the specific example for the central node 210 in FIG. 2,however, the optical amplifier 212 in the fiber ring 202 is, in additionto its optical amplification function, used and operated as an opticalgate switch to provide a default optical break point in the ring 202. Inoperation, the optical amplifier 212 is turned off to create the defaultbreak point under a normal operation condition and is tuned on to closethe default break point and thus operates as an amplifier when there isa break point elsewhere in the ring network 200. The other opticalamplifier 217 in the fiber ring 201 in the central node 210 generallyoperates as a “regular” optical amplifier.

In some ring networks, two or more adjacent nodes may be separated fromone another over a relatively short fiber span and the optical loss overthis short fiber span is sufficiently small so that at least one ofthese adjacent nodes does not need to have an optical amplifier in eachof the two fiber rings. Such a node without an optical amplifier can beeither a central node or a regular node. In absence of the opticalamplifier, this node can use an optical switch, e.g., an 1×1 opticalswitch, to provide the local optical switching function in this node tocreate a protective optical break point as part of the opticalprotection switching.

The above use of the amplifier as the gate switch 212 in the centralnode 210 is efficient because a separate switch for the gate switch 212is eliminated. When the gate switch 212 in the central node 210 isclosed, an amplifier in the fiber ring 202 within the central node 210can be used to balance the signal strengths out of the central node 210in both fiber rings 201 and 202. An amplifier as the gate switch 212 canperform both optical switching function and the optical amplificationfunction. As an alternative, the gate switch 212 in the fiber ring 202within the central node 210 may be implemented by a combination of anoptical amplifier and a real optical gate switch like the optical switch211 in the fiber ring 201, or simply an optical switch without opticalamplifier if the optical gain is not needed. In addition, a combinationof an optical amplifier and a variable optical attenuator (VOA) with asufficient extinction ratio and fast switching time may be used as thegate switch 212. An VOA can be configured and operated to exhibit a highoptical attenuation and a low optical attenuation and can be adjusted tooperate at a variable attenuation level between the high and the lowattenuation levels. When the high attenuation in the VOA is sufficientlyhigh to suppress the optical transmission below a desired level, thetransmission of the VOA is “darkened” or deactivated and thus canoperate like an optical switch in an open position. Certain opticalswitching operations described in this application are based onswitching operation of a VOA.

In FIG. 2, each node is designed to place each optical amplifier (217,216 or 212) coupled in one fiber ring 201 or 202 at a location upstreamfrom a broadband coupler for dropping WDM signals from the ring into thenode. In the central node 210, for example, the optical amplifier andgate switch 212 in the line card 210A is located upstream from thebroadband coupler in the clockwise fiber ring 202 that splits thetraffic in the ring 202 to produce optical drop signals 207A. The otheroptical amplifier 216 in the line card 210B of the central node 210 islocated upstream from the broadband coupler in the counter clockwisefiber ring 201 that splits the traffic in the ring 201 to produceoptical drop signals 207B.

In addition, corresponding to each optical amplifier (217, 216 or 212)coupled in one fiber ring within each node, a VOA 214 is placed in theother fiber ring within the same node at a location downstream from abroadband coupler for adding one or more WDM signals from the node ontothe ring. In the central node 210, for example, the VOA 214 in the linecard 210A is located downstream from the broadband coupler in thecounter clockwise fiber ring 201 that adds one or more optical addsignals 206A to the ring 201 and is upstream from the gate switch 211.The other VOA 214 in the line card 210B of the central node 210 islocated downstream from the broadband coupler in the clockwise fiberring 202 that adds one or more optical add signals 206B to the ring 202.

Under this node design, each node has an upstream optical amplifier atthe entrance of the node to amplify received optical signals in eachfiber ring and a downstream VOA at the exit of the node in each fiberring to control the optical power of each signal going out of the node.Signal adding and dropping functions are implemented in each nodebetween the upstream optical amplifier and the downstream VOA. Inaddition, a wavelength-selective optical coupler 215 is coupled at anupstream location from the upstream optical amplifier in each node toselectively split a portion of the circulating probe signal fordetecting the circulating probe signal at the entrance of the node. Thisuse of the VOA and the detection design in each node allow thecirculating optical probe signal at the probe wavelength, the added DWDMwavelengths or upstream ASE noise to propagate through a repaired fiberbreak to a downstream node and be detected by a detector in thedownstream node. As a result, when a fiber break is repaired, the systemcan be automatically informed of the status of the repair without humanintervention and can automatically revert the system back to the normaloperation after the repair is completed. These and other features in thenode design can be used to achieve various operations associated withthe protection switching based on the optical circulating probe signal.

Both gate switches 211 and 212 are in the line card 210A of the centralnode 210. The gate switch 211 is downstream from the location where oneor more added WDM signals are added to the fiber ring 201 in the counterclockwise direction and is upstream from the location where thecirculating optical probe is added to the fiber ring 201 in the counterclockwise direction. The gate switch 212 is upstream from the locationwhere one or more added WDM signals are added to the fiber ring 202 inthe clockwise direction and the location where the circulating opticalprobe is added to the fiber ring 202 in the clockwise direction.Alternatively, both gate switches 211 and 212 can be implemented in theline card 210B of the central node 210.

The generation and detection of the circulating optical probe signal areimplemented as follows in the network 200 in FIG. 2. The optical probesignal in each direction is generated from the probe transmitter 203inside the central node 210. The probe transmitter 203 can be a laseroperable to produce laser light at a desired probe wavelength. Anoptical splitter or coupler 204 can be used to split the optical probesignal into a first optical probe signal and a second optical probesignal that are respectively launched into the two counter propagatingdirections in the network 200. Two optical probe couplers arerespectively coupled to the fiber rings 201 and 202 to couple the firstand second optical probe signals onto the fiber rings 201 and 202 inopposite directions. Each optical probe coupler is located in each fiberring downstream from a respective gate switch in the central node 210.In the example in FIG. 2, the optical probe coupler for coupling theoptical probe signal to the counter clockwise ring 201 is downstreamfrom the gate switch 211 in the line card 210A so that the switchingoperations of the gate switch 211 do not affect coupling of the opticalprobe signal to the ring 201. Similarly, the optical probe coupler forcoupling the optical probe signal to the clockwise ring 202 isdownstream from the optical amplifier as the gate switch 212 so that theswitching operations of the gate switch 212 do not affect coupling ofthe optical probe signal to the ring 202.

The loss of the optical probe signal at a particular location in thering network 200 in FIG. 2 can be caused by either an optical break inthe ring or the failure of the probe transmitter 203. These two causesneed be handled differently. In order to distinguish the loss of theoptical probe signal caused by an optical break in the ring from theloss of the optical probe signal caused by the failure of the probetransmitter 203, a probe transmitter photodetector can be implemented tomonitor the optical output of the probe transmitter 203 and thus detectwhether the probe transmitter 203 generates the probe light. In oneimplementation, a beam splitter is placed in the optical path of theoptical output of the probe transmitter 203 to split a small fraction ofthe probe light into the probe transmitter photodetector. In anotherimplementation, the probe transmitter photodetector can be aphotodetector at the back facet of the laser diode used as the probetransmitter 203 to monitor laser light output at the back facet of thelaser diode. The detected signal from the probe transmitterphotodetector can be used by the control unit of the central node 210,such as the element management system (EMS), to control the protectionswitching mechanism. When the probe transmitter photodetector indicatesa failure of the probe transmitter 203, the loss of the optical probesignal in other nodes of the ring is not treated as an optical break andthe protection switching is not effectuated. A service request isgenerated for repairing or replacing the failed probe transmitter 203.When the probe transmitter photodetector indicates a normal operation ofthe probe transmitter, the loss of the optical probe signal in the ringtriggers the protection switching mechanism.

A back-up probe transmitter may be provided for backing up the probetransmitter 203 and is turned on to produce the first and the secondoptical probe signals when the default probe transmitter 203 fails.Alternatively, two separate probe transmitters may be used to producethe first and the second optical probe signals, respectively, for thetwo directions of the ring and each of the two probe transmitter may bereplicated for redundancy.

A first optical probe coupler is coupled to the fiber ring 201 at alocation downstream from the gate switch 211 in either line card 210A orline card 210B to add the first optical probe signal onto the fiber ring201 to propagate in the counter clockwise direction. The first opticalprobe coupler on the fiber ring 201 is designed to allow the optical WDMsignals and first optical probe signal to pass through in the fiber ring201. A second optical probe coupler is coupled to the fiber ring 202 ata location downstream from the gate switch 212 in either line card 210Aor line card 210B to add the second optical probe signal onto the fiberring 202 to propagate in the clockwise direction. Similar to the firstoptical probe coupler, the second optical probe coupler on the fiberring 202 is also designed to allow the optical WDM signals and secondoptical probe signal to pass through in the fiber ring 202. Hence, undera normal operating condition when each of the optical gate switches 211and 212 is open to create a single default break point in each of thetwo fiber rings 201 and 202, the optical probe signal, i.e., each of thefirst and the second optical probe signals, circulates the entire fiberring except for the default break point.

Notably, in the illustrated centralized control mode, each node isdesigned to selectively drop a portion of each of the first and thesecond optical probe signals at the entrance of each node for each fiberwhile allowing a remainder of each of the first and the second opticalprobe signals to continue on in the optical ring network 200. Hence,each optical probe signal circulates in the entire ring and is used todetect a break point caused by optical failure outside the central node210. This aspect of the central node 210 and a regular ring node 220 isdescribed below.

In the central node 210, a wavelength-selective optical coupler 215,such as an optical filter, is coupled at an upstream location from thegate switch 212 and located at near the entrance of the central node 210in the fiber ring 202 where the light propagates in the clockwisedirection. The wavelength-selective optical coupler 215 selects andsplits a small portion of the second optical probe signal from the fiberring 202 to drop as a local probe monitor signal while allowing alloptical WDM signals and the majority of the second optical probe signalto pass through. A probe detector PD1 is used to detect the droppedlocal probe monitor signal. This allows for detecting the presence ofthe second optical probe signal at the entrance to the central node 210in the fiber ring 202. Similarly, another wavelength-selective probecoupler 215 and a respective probe detector PD1 are coupled at anupstream location of the optical amplifier 217 in the fiber ring 201 todetect the presence of the first optical probe signal at the entrance tothe central node 210 in the fiber ring 201.

FIG. 2A shows an example of the wavelength selective coupler 215 in FIG.2 for selectively dropping a portion of the circulating optical probesignal in each node. The wavelength selective coupler 215 includes afiber tap coupler 215A coupled on the main fiber line in either fiberring 201 or fiber ring 202 to drop a portion of light at all wavelengthsand a probe bandpass filter 215B in the optical path of the droppedlight from the fiber tap coupler 215A to select only light at the probewavelength for detection by the optical probe detector PD1 whilerejecting light at other wavelengths.

Turning now to the regular ring node 220, a wavelength-selective opticalcoupler 215 is coupled at an upstream location from the amplifier 215 inthe fiber ring 201 where the light propagates in the clockwise directionand enters the node 220. A probe detector PD1 is used to receive adropped local probe monitor signal from the fiber ring 201 and to detectthe presence of the first optical probe signal at the entrance to thering node 220 in the fiber ring 201. Similarly, anotherwavelength-selective probe coupler 215 and a respective probe detectorPD1 are coupled at an upstream location of the optical amplifier 217 inthe fiber ring 202 to detect the presence of the second optical probesignal at the entrance to the ring node 220 in the fiber ring 202. Otherring nodes such as the ring node 230 have a similar design for couplingand detecting the optical probe signal. Backup units for the above probetransmitter 203 and the probe detectors PD1 may be used to provideredundancy for the generation and detection of the circulating opticalprobe signal and to improve the reliability of the protection switching.

In addition to the probe detectors PD1, each node can include anoptional optical detector PD2 coupled to the fiber line at a locationdownstream from each amplifier within the node as an optical monitor forthe optical amplifier. An optical coupler can be used to couple afraction of the light in the fiber line into the optical detector PD2.When the optical amplifier upstream from the optical detector PD2 inthat node fails, the optical detector PD2 detects a loss of light belowa predetermined threshold level. The failure of the optical amplifier inthis node shuts down all optical signals passing through the failedoptical amplifier and thus triggers the protection switching mechanismdue to the detected loss of the optical probe at one or more nodes inthe ring. The detected result from each optical detector PD2 can bereported to the central node 210 via a node-to-node communicationmechanism such as the OSC signaling described above. The loss of lightdetected at the detector PD2 may be caused by an upstream optical breakother than the failure of the optical amplifier, e.g., a device failurein an upstream node or a fiber cut at an upstream location. The centralnode 210 can determine the cause for the loss of signal at the PD2 basedon received information based on detection data obtained at otherdetectors and other nodes.

A second optional optical detector PD3 may also be coupled to a fiberring downstream from a VOA 214 in each node to monitor the VOA 214. Anoptical coupler can be used to couple a fraction of the light in thefiber line into the optical detector PD3. When the VOA 214 upstream fromthe optical detector PD3 in that node fails, the optical detector PD3detects a loss of light below a predetermined threshold level and thisfailure also triggers the protection switching mechanism. The detectedresult from each optical detector PD3 can be reported to the centralnode 210 via a node-to-node communication mechanism such as the OSCsignaling described above. The loss of light detected at the detectorPD3 may be caused by an upstream optical break other than the failure ofthe VOA, e.g., the failure of the upstream amplifier in the same node, afailure in an upstream node or a fiber cut at an upstream location. Thecentral node 210 can determine the cause for the loss of signal at thePD3 based on received information based on detection data obtained atother detectors and other nodes.

With the above design in FIG. 2, each node can be operated to monitorthe circulating optical probe, signal at the “in band” probe wavelengthby monitoring the dropped probe monitor signal at each PD1 to determinewhether the first and the second optical probe signals are present. Thedetected status of each of the first and second optical probe signals ineach node is then transmitted from each regular ring node to the centralnode 210 by using the OSC signaling between two neighboring nodes. Thecentral node 210 collects the reported data on status of the circulatingoptical probe signal from the nodes in the ring network 200 andprocesses the reported data to decide (1) whether there is a break pointand (2) if so, the location of the break point. If no node reports lossof the either of the first and second optical probe signals, the centralnode 210 keeps both gate switches 211 and 212 open in the fiber rings201 and 202, respectively, and to keep other ring nodes at their normaloperation modes where each optical amplifier 217 is turned on oractivated to amplify light of the optical WDM signals and thecirculating first and second optical probe signals carried in the fiberrings 201 and 202 and each VOA 214 is set at an appropriate attenuationlevel to allow optical transmission of the optical WDM signals and thecirculating first and second optical probe signals. FIG. 2 illustratesthe normal operation mode for each node.

When there is a break point at a location outside the central node 210in either one or both of the two fiber rings 201 and 202, at least onenode including the central node 210 can detect the loss of the opticalprobe signal (either or both of the first and the second optical probesignals). From the status of the optical probe signal from all nodes,the central node 210 can determine the location of the break point. Forexample, if the break point in the fiber ring 201 occurs between thenodes 220 and 230, the dropped probe monitor signals for dropping aportion of the first optical probe signal become absent from the probedetectors PD1 coupled to the fiber ring 201 in the node 230 and thecentral node 210. All other probe detectors PD1, including all probedetectors PD1 in the fiber ring 202, still detect the presence of theirrespective optical probe signals. Hence, the reported data from thenodes via the OSC signaling to the central node 210 provides a “probestatus map” which can be processed by the protection switching controlsoftware or hardware logic in the central node 210 to determine thelocation of the break point. Detectors PD2 and PD3, if implemented, canbe reported to the central node 210 to provide more detailed informationregarding the nature of the failure and such information can facilitatedetermining the location and exact nature of the failure.

After the break point between two regular ring nodes or within a ringnode (e.g., a failure of an optical amplifier, a failure of a jumperbetween line cards, or a VOA failure inside a node) is detected, e.g., abreak point between the nodes 220 and 230, the central node 210activates the protection switching mechanism to close the two defaultbreak points at the gate switches 211 and 212 inside the central node210 and send commands using the OSC signaling to cause one protectiveoptical break point in each of the two ring nods to cut off light inboth the first and the second directions, e.g., in both fiber rings 201and 202. Hence, optical communication traffic in both directions thatgoes through the location of the break point is cut off and is re-routedthrough the central node 210 by closing the default break point in eachof the two directions, e.g., the two fiber rings 201 and 202. After thebreak point is repaired, the default break point in the central node 210is restored and the protective optical break point in each of the tworing nodes is closed to restore transmission of light through the tworing nodes. During this process, the circulating optical probe signal ismonitored at all nodes to provide the connection status of the networkto the central node 210.

In the ring network 200 under the centralized control design, both gateswitches 211 and 212 can be synchronized to open or close together. Theswitching states depend whether the probe wavelength is present (bothgate switches open) or absent (both gate switches closed). The loss ofthe optical probe signal can be caused by a fiber cut between nodes or anode failure such as a failed VOA or amplifier inside a node. Thefollowing sections describe examples of protection switching in the ringnetwork 200.

FIGS. 3A, 3B, 3C and 3D show an example of the protection switchingoperations in the dual-fiber ring network 200 in FIG. 2 when both fiberrings 201 and 202 are cut and later repaired. In FIG. 3A, after thefiber rings 201 and 202 are cut at a location between the ring nodes 220and 230, the first and second optical probe signals are lost at theprobe detectors PD1 in both fiber rings 201 and 202 within the centralnode 210. This local event within the central node 210 triggers thecentral node 210 to close the two gate switches 211 and 212. When thegate switch 212 is an amplifier as illustrated, the amplifier is turnedoff under a normal condition and is now turned on to close the gateswitch when a fiber cut is detected. This action in the central node 210is taken based on a local detection of the optical probe signal byeither one of the two probe detectors PD1 within the central node 210.This action allows the optical traffic that is blocked by the fiber cutbetween the nodes 220 and 230 to reroute via the central node 210 andthus communications in the ring network 202 can be maintained. At thistime and prior to all status reporting data from other nodes isreceived, the central node 210 does not know the location of the opticalfailure caused by the fiber cut.

The nodes 210 and 220 report their status to the central node 210 viathe OSC signaling. The status of nodes 220 and 230 is as follows. Thering node 220 still detects the first optical probe signal at its probedetector PD1 in the fiber ring 201 but loses its second optical probesignal in its probe detector PD1 in the fiber ring 202. Any ring nodebetween the central node 210 and the node 220, if present, would havedetected the same probe signal status as in the node 220. In the ringnode 230, the first optical probe signal in the fiber ring 201 ismissing at the probe detector PD1 in the fiber ring 201 while the secondoptical probe signal is present. Similarly any node present between thecentral node 210 and the node 230 would have the same probe signalstatus as in the node 230. Hence, after the status reporting data fromthe nodes is received, the central node 210 can process the data anddetermine the location of the optical failure to be between nodes 220and 230.

Based on this determination, the central node 210 sends out commands,via the OSC signaling, to the nodes 220 and 230 to create a protectivebreak point in each fiber downstream from the location of the fiber cutto block any optical traffic through the fiber span between the nodes220 and 230 even if the fiber cut is repaired. In the fiber ring 201,the downstream node from the location of the fiber cut is the node 220and thus the optical amplifier 217 in the fiber ring 201 within the node220 is turned off or deactivated to create a protective break point inthe fiber ring 201. In the fiber ring 202, the downstream node from thelocation of the fiber cut is the node 230 and thus the optical amplifier217 in the fiber ring 202 within the node 230 is turned off ordeactivated to create a protective break point in the fiber ring 202.This is shown in FIG. 3B. Notably, the optical communication elsewherein the ring network 200 is maintained in FIG. 3B. The use of theamplifier in the downstream node from the fiber cut in each fiber forthe protection switching allows the probe detector PD1 to automaticallydetects the status of the upstream fiber cut and when the fiber cut isrepaired by monitoring the optical probe signal at all time and thusnotifies the central node 210.

Next, the fiber cut can be repaired by dispatching a field repair personor crew. During the repair process, the protection switching conditionshown in FIG. 3B remain unchanged. FIG. 3C shows an example where onefiber cut in the fiber ring 202 is repaired while the fiber cut in thefiber ring 201 still remains. This condition is reported to the centralnode 210 via the OSC signals sent from the nodes 220 and 230. Under thiscondition, the gate switches 211 and 212 in the central node 210 remainclosed to continue rerouting the optical traffic through the centralnode 210 and the deactivated amplifiers in the nodes 220 and 230 remaindeactivated to block optical traffic between the nodes 220 and 230. Thisstatus of the network 200 is shown in FIG. 3C. Due to the node structureillustrated in FIG. 2, an optical amplifier 217 in a node downstreamfrom the fiber break is turned off and is thus operated as an opticalopen switch to block the transmission. At this time, a VOA 214 in anadjacent node upstream from the fiber break remains at an opticaltransmissive state without completely blocking light. This use of theVOA in the upstream node allows the downstream neighbor node to detectwhether or not a fiber cut is repaired by detecting the presence of theprobe wavelength via the wavelength selective coupler 215 and the probedetector PD1. In a normal operation condition, a VOA 214 can be adjustedsuch that the optical power per wavelength at the input to the opticalamplifier in the downstream neighbor node is at around −25 dBm orhigher, and the probe wavelength power could be the same or slightlylower. As an example, the detected probe wavelength power at the opticaldetector PD1 under a normal operation condition can be around −35 dBm(assuming a tap coupler with a 10% coupling ratio is used to split 10%of the light to the detector PD1), as shown in PD1 at the node 220 inFIG. 3C. This will be the case when a fiber cut is repaired (or whenthere is no fiber cut) and a minimum of −35 dBm is detected: If thefiber cut is not repaired, the probe light is completely blocked by thefiber cut, and the detected power at the probe detector PD1 is thedetector's sensitivity limit, e.g., −47 dBm in the example shown in thenode 220 in FIG. 3C.

After the fiber cuts in both fiber rings 201 and 202 are repaired asshown in FIG. 3D, the nodes 220 and 230 inform the central node 210 ofthis status by communicating the detected results at the probe detectorsPD1 in nodes 220 and 230 to the central node 210 via the OSC signaling.Based on this information, the central node 210 sends out commands tonodes 220 and 230 to turn on the amplifiers in nodes 220 and 230 toremove the protective break point in each direction. After the nodes 220and 230 receive the commands and turn on their respective amplifiers,the entire fiber ring is a closed loop in each direction and hence theoptical probe signal, which is launched in the central node 210, can nowcirculate back to the central node 210 in both directions. The centralnode 210 can detect the presence of the optical probe signal in bothfiber rings 201 and 202 locally and this event triggers the central node210 to open the gate switches 211 and 212 to restore the default breakpoint in each fiber ring. At this time, the network 200 is reverted backto its normal operating state shown in FIG. 2.

The above protection switching provides an automatic reverted switchingto the default protection switching state where a break point in eachdirection is maintained at the central node. Based on the location ofthe optical failure, the proactive break point or points can vary inlocation but after the optical failure is corrected, the break point ineach direction is restored back to the central node.

FIGS. 4A, 4B and 4C illustrate a part of the protection switchingsequence in the network 200 in FIG. 2 when only one fiber ring 202 iscut between the nodes 220 and 230.

The protection switching is the same by closing the default break pointin both fiber rings 201 and 202 and creating the protective break pointsin both fiber rings in nodes adjacent to the location of the fiber cuteven though the other fiber ring 201 does not experience any opticalfailure.

The optical amplifier used as the gate switch 212 in the central node210 and other optical amplifiers 217 in the nodes 220 and 230 need tohave a fast response to switch between on and off due to the requirementof protection switching of at least 50 msec in current commercialnetworks. Such a high-speed switching optical amplifier may be replacedby a combination of a fast optical switch with an optical amplifierwithout the fast switching capability. In addition, a fast switchingvariable optical attenuator and an optical amplifier without the fastswitching capability may also be used to replace each high-speedswitching optical amplifier in FIG. 2.

The protection switching described above for a fiber cut between nodescan be similarly applied to protection switching for a device failure ina node by treating the device failure as a fiber cut in a nearest fiberspan to the failed device. Referring to FIG. 2, the failure of theoptical amplifier 217 in the line card 220A within the node 220 istreated as a fiber cut in the fiber ring 202 between the nodes 220 and230 and thus the amplifier 217 in the fiber ring 201 in the oppositedirection in the line card 230B of the nearest node 230 is shut downalong with the gates switches 211 and 212 in the central node 210 in theprotection switching. This switching can be triggered by the localdetection by the detectors PD2 and PD3. The corresponding operatingcondition after this switching is identical to what is in FIG. 3B. Asanother example, the failure of the VOA 214 in the line card 220B withinthe node 220 is treated as a fiber cut in the fiber ring 202 between thenodes 220 and 210 and thus the amplifier 217 in the fiber ring 201 inthe opposite direction in the nearest node 220 is shut down along withthe gates switches 211 and 212 in the central node 210 in the protectionswitching. As yet another example, if the optical amplifier 217 in theline card 210B of the central node 210 fails, the amplifier 217 in theline card 230A of the reverse direction neighbor node 230, which isclosest to the failed amplifier, is turned off Hence, for non-centralnodes, after a fiber break or device failure, under the control of thecentral node 210, a switch (either an amplifier or a regular 1×1 switch)downstream from the fiber break or failed device is opened, the VOA inthe reverse direction and in the same card is darkened so that the otherswitch in a downstream neighbor node in the reverse fiber direction anddownstream from the fiber break or failed device is also opened. At thesame time, the fiber break or device failure causes loss of the opticalprobe signal in at least one of the fibers in the central node 210 andthis causes the central node 210 to close the gates switches 211 and212.

In the centrally controlled ring network 200 in FIG. 2, the statusreporting from a ring node to the central node 210 and the delivery of acommand from the central node 210 to a ring node are based on thenode-to-node optical communication using an out of band opticalsupervision channel. Alternatively, the circulating optical probe signalcan be used and modulated to carry the data for the status reportingfrom each node to the central node and delivery of commands from thecentral node to each node for execution of the protection switchingdescribed above. This use of the circulating optical probe signal caneliminate the need for the OSC signaling in the protection switching.Accordingly, the detection of the optical probe signal can includeprocessing of the modulated data in the optical probe signal beyond thesimple detection of the power level of the circulating probe signal atthe optical probe detector PD1.

In other implementations, the circulating optical probe signal can bemodulated to carry other control and management data that may becommunicated via OSC signaling. This use of the circulating opticalprobe signal can completely eliminate the need for the OSC signaling andOSC transmitters and receivers and other associated hardware in the ringnetwork and thus can simplify the node structure, the ring structure,and the operations of the nodes and the ring. Various modulationtechniques may be used in each node to modulate the optical probesignal. An optical modulator may be added in each node to modulate theoptical probe signal. An existing VOA in a non-central node can also beused to modulate the probe signal by controlling the attenuation of theVOA to superimpose reporting data to the central node.

As an alternative to the above switching protection mechanisms based oncentrally controlled ring networks, each regular node in a ring may bedesigned to control its local switching operations entirely based onlocal information within the node without the wait for a command fromthe central node 210. A hybrid mode protection switching can be providedto combine the central protection switching in the central node based onthe circulating optical signal and local switching in each non-centralnode based on local information without the control from the centralnode.

In one implementation of this hybrid mode protection switching, thecentral node uses its local detection of the circulating optical probesignal to operate the local gate switches within the central node toopen or close the default break point in each direction based on thedetected status of the circulating optical probe signal in the centralnode. In this aspect, the hybrid mode is similar to the centralized modein the network in FIG. 2. However, different from the centralized modein FIG. 2, the central node in the hybrid mode no longer controls allother regular ring nodes for the protection switching. Instead, otherregular ring nodes are self controlled and operate their respectivelocal switching to generate a local protective break point based on alocal detection of the light in the network without relying oninformation or commands from the central node. Notably, each non-centralnode does not report the local node condition to the central node inorder to respond to a fiber break or device failure. Neither does eachnon-central node wait for a control command from the central node beforetaking any action in response to a fiber break or device failure. Withthe exception of a non-central node adjacent to the central node on theside of the central node with gate switches, this hybrid mode protectionswitching eliminates the communications between the central node andeach non-central node via the OSC signaling in activating the protectionswitching by creating a local optical break point after occurrence of afiber cut or device failure.

In a hybrid mode protection switching, the optical nodes in the ring areclassified into three categories and are configured and operateddifferently: the central node, a special non-central node next to thecentral node on the side of the central node with gate switches, andother regular ring nodes. Different from the centralized mode, thecirculating optical probe in each of the two counter propagatingdirections is monitored separately from other optical signals in thecentral node only and a regular node monitors the total optical power ineach fiber ring. Like the central node in the centralized mode, thecentral node uses an optical coupler and a probe-wavelength filter,e.g., the design in FIG. 2A, to split a portion of the circulatingoptical probe signal in each of the two directions as a monitor signal.In its default state, the central node maintains a default break pointin each of the two directions in the ring when the circulating opticalprobe signal is present in both directions of the ring. When the opticalprobe signal in either one of the two directions is not detected, thecentral node operates to close the default optical break point in eachof the two directions and open the default optical break point when theoptical probe signal in each of the two directions is detected. In aregular ring node that is not the central node, a total optical power oflight received in the respective ring node is monitored, withoutmonitoring the circulating optical probe signal separately, to determinewhether the total optical power is below a power threshold associatedwith a fiber break. In implementations, this power threshold can be setat a low value so that the received total optical power at a node thatis not immediately downstream from a fiber cut is above this powerthreshold due to light from an immediate upstream node such as one ormore added WDM channels or the amplified spontaneous emission (ASE) fromthe upstream node. This design can be used to ensure that onlynon-central nodes next to a fiber cut are triggered to react to thefiber cut while other non-central nodes remain in their normaloperation.

In one implementation, the special non-central node next to the side ofthe gate switches of the central node can operate differently from otherregular ring nodes and rely on the OSC signaling with the central nodeto perform its location protective switching when the detected totaloptical power is below the power threshold. For Other regular ringnodes, each ring node creates a protective optical break point when thetotal optical power is below the power threshold. Hence activation ofthe protection switching can be achieved without relying on informationor commands from the central node and any other ring nodes. The OSCsignal, for example, is no longer needed for activation of theprotection switching in these nodes. Hence, this hybrid control mode canreduce the need for node-to-node communications in protection switchingand essentially eliminate the dependency of the response time for theprotection switching on the number of nodes in the ring, and thus canprovide a fast response time in protection switching.

FIG. 5 shows one example of a dual-fiber ring network 500 in a hybridcontrol mode. Examples of the nodes shown are a central node 530 (node3) and four non-central ring nodes 510 (node 1), 520 (node 2), 540 (node4) and 550 (node 5). The central node 530 in FIG. 5 operates in a waysimilar to the operation of the central node in the centralized mode inFIG. 2: the central node 530 separately monitors the circulating opticalprobe signal and controls the gate switches 211 and 212 based on thestatus of the detected circulating optical probe signal.

The node 520 next to the central node 530 is a special node that isdifferent from other non-central nodes because it is located on the sideof the gate switches 211 and 212 of the central node 530. Under thenormal operating condition where there is no other fiber cut or nodefailure in the ring network 500, the gate switches 211 and 212 are opento create default breaks in the fiber rings 202 and 202. Other than theOSC and circulating optical probe signals, the WDM signals and ASE fromthe amplifier 217 in the fiber ring 201 are blocked by the default breakof the gate switch 211. The total optical power received by the specialnode 520 in the fiber 201 may be below the power threshold under thenormal operating condition. This relation makes the node 520 a specialnon-central node different from other non-central nodes 540, 550 and510. To avoid shutting down the special node 520 under the normaloperating condition, the special node 520 can be controlled to detectthe fiber cut in the fiber span from the special node 520 to the centralnode 530 based on a node-to-node communication between the nodes 520 and530 and to operate its protection switching based on the node-to-nodecommunication. This node-to-node communication can be achieved by, e.g.,the OSC signaling, modulation of the optical probe signals and others.When the circulating optical probe signal is modulated to carry networkcontrol and management data, the hardware for OSC signaling includingOSC transmitters and receivers can be eliminated. Various modulationtechniques may be used in each node to modulate the optical probesignal. An optical modulator may be added in each node to modulate theoptical probe signal. An existing VOA in a non-central node can also beused to modulate the probe signal by controlling the attenuation of theVOA to superimpose reporting data to the central node. In the followingexamples, the node-to-node communications used in the hybrid protectionswitching is the OSC signaling and the same process in these examplescan be applied to node-to-node communications based on modulated opticalprobe signals. Whenever there is a loss of the OSC signaling between thespecial node 520 and the central node 530 in at least one of the fiberrings 201 and 202, the special node 520 opens up the two switches (i.e.,optical amplifiers 217) in the fiber rings 201 and 202 to block opticaltransmission through the special node 520.

Other non-central regular nodes 540, 510 and 550 are operated entirelybased on their local detection of the total optical power withoutrelying on communications with the central node 530 and other nodes.

Some aspects of the nodes in FIG. 5 are similar to the ring network 200in FIG. 2 while other aspects are different. Similar to the nodes inFIG. 2, each node in FIG. 5 can be implemented with two amplifier linecards and can use the optical amplifiers as optical switches to createoptical breaks when the protection switching is activated. A node mayalso replace the amplifiers by 1×1 optical switches when the opticalgain in that node is not needed when, e.g., the node is connected in ashort fiber span between two other nodes with amplifiers. The centralnode 530 in the example shown in FIG. 5 has two gate switches 211 and212 in the line card 210A that interfaces with the non-central node 520(node 2). The features in the central node 530 for coupling the opticalprobe signals in both fiber rings 201 and 202 and the local detection ofthe optical probe signals are similar to the central node 210 in FIG. 2.For example, a wavelength-selective coupler 215 is used in the centralnode 530 to selectively drop a fraction of the optical probe signalwhile transmitting light at the WDM signal wavelengths so that eachprobe detector PD1 coupled to the coupler 215 is designated to measureonly the optical probe signal. Based on the local status of the opticalprobe signals in both fiber rings 201 and 202, the central node 530operates the gate switches 211 and 212 to open or close default opticalbreak points in the fiber rings 201 and 202.

For the regular ring nodes 540, 550 and 510, each wavelength-selectivecoupler 215 in the ring node in FIG. 2 that is placed at the entrance ofeach ring node in each fiber ring to selectively split a portion of thecirculating optical probe signal is now replaced by a broadband couplerthat splits a fraction of all optical signals in each fiber ring to thedetectors PD1 and the split light by the broadband coupler includes theoptical WDM signals, the circulating optical probe signal and ASE.Hence, in the ring network 500 in FIG. 5, each detector PD1 measures thetotal optical power in each fiber ring received by the ring node and thelocation of the broadband coupler is upstream from the coupler that addsone or more WDM add signals. A predetermined power threshold power levelis used by each ring node to determine whether there is an optical breakpoint at a location upstream from the ring node. If the detected opticalpower is below the threshold power level in a first fiber ring, the ringnode reacts to shut off its optical transmission in the first fiber ringand the optical transmission in the other, the second fiber ring. Thisaction triggers the downstream node in the second fiber ring, which isthe upstream node in the first fiber ring, to turn off its transmissionin both the first and the second fiber rings in the same manner. Whenthe measured optical power at each detector PD1 is no longer below thepower threshold, the optical transmission in each ring node resumesunder the control by the central node 530.

Each regular ring node in FIG. 5, similar to a regular ring node in FIG.2, has a VOA 214 downstream from the optical amplifier 217 and thedetector PD1 in each fiber ring. Different from the regular ring in FIG.2, each regular ring node in FIG. 5 further uses each VOA 214 as anoptical switch in the optical protection switching. In each regular ringnode in FIG. 5 (e.g., the node 510), when the optical detector PD1 in afirst fiber ring (e.g., the ring 202) detects a loss of light, thecorresponding switch immediately downstream from the detector PD1 in thefirst fiber ring 202, e.g., optical amplifier switch 217, is turned offto block 20, optical transmission while the ring node 510 opens up acorresponding switch (VOA 214) in the second fiber ring 201 within thesame node to turn off its optical transmission at the output of the samenode in the second fiber ring 201. This action shuts down opticaltransmission in both fiber rings 201 and 202 through the node 510. Thisaction further triggers an immediate downstream node 550 in the secondfiber ring 201, which is the immediate upstream node in the first fiberring 202 on the other side of the fiber break, to detect the loss of theoptical power in the second fiber ring 201 and thus opens up its localoptical switches, i.e., the optical amplifier 217 in the second fiberring 201 and the VOA 214 in the first fiber ring 202. Next, the upstreamVOAs 217 respectively in the two nodes 510 and 550 in the two fiberrings 201 and 202 are controlled to restore their normal opticallytransmissive operating mode to allow optical transmission while the twoturned-off downstream optical amplifiers 217 respectively in the nodes510 and 520 in the in the two fiber rings 201 and 202 remain turned offto block optical transmission. Under this condition, when the fiberrepair is detected via the photo-detector PD1 in nodes 510 and 550 nearthe fiber break and the nodes 510 and 550 can be automatically restoredto their normal operations after the repair is completed based on suchoptical detection downstream from the fiber break. The above localprotection switching without communication with the central node 530 canbe used to respond to a fiber cut either in only one of the two fiberrings 201 and 202 between the nodes 510 and 550 or in each of the twofiber rings 201 and 202 between the nodes 510 and 550.

FIGS. 6A, 6B, 6C and 6D illustrate an exemplary protection switchingsequence in the ring network 500 in FIG. 5 when both fiber rings 201 and202 are cut at a location between regular ring nodes 510 and 550. Undera normal condition, the gate switches 211 and 212 in the central node530 are open to create the default optical break point in both fiberrings 201 and 202 and all other ring nodes transmit light.

When the fiber cut occurs in both fiber rings 201 and 202 between nodes510 and 550, the detector PD1 in the node 510 downstream from thelocation of the fiber cut in the fiber ring 202 and the other detectorPD1 in the node 550 downstream from the location of the fiber cut in thefiber ring 201 detect the loss of optical power. In each of the nodes510 and 550, the VOA 214 upstream from the fiber cut in one fiber ringand the optical amplifier 217 downstream from the fiber cut are operatedto block optical transmission. At the same time, the central node 530detects loss of the optical probe signal in both fiber rings 201 and 202after the fiber cut occurs and thus closes the gate switches 211 and 212to allow optical transmission through the central node 530 to re-routethe optical traffic blocked by the fiber cut between the nodes 510 and550. Other nodes, such as nodes 520 and 540, do not detect any abnormalconditions based on the received total optical power in each fiber ringand remain in their normal operation to continue to transmit light inboth directions. This is shown in FIG. 6A. The node 520, for example, isnot next to the fiber cut between the nodes 510 and 550 and receives theoptical power from the added WDM signals from an upstream node, e.g.,the central node 530 as an upstream node and other upstream nodes 540and 550 in fiber ring 201, and the node 510 as an upstream node in thefiber ring 202. In addition, the node 520 could also receive opticalpower from amplifier ASE noise from each upstream node that is not shutdown, e.g., there may be a booster optical amplifier present at the exitof the node 510 in fiber 202. The threshold power in each node can beset at a level to be less than the total optical power of the ASE noiseand added WDM signals from an upstream node to ensure only the two nodesnext to the fiber cut activate their local protection switching when thefiber cut occurs.

The blocking of optical transmission in the VOAs 214 in the nodes 510and 550 shown in FIG. 6A prevents each local detector PD1 downstreamfrom the fiber cut to monitor whether optical transmission is restoredin each fiber ring between the nodes 510 and 550. Next shown in FIG. 6B,after the above switching operations in the nodes 510, 550 and 530 arecompleted, the node 510 turns the VOA 214 in the other fiber ring 201back to its normal attenuation value to allow for optical transmission.The node 550 performs the same switching operations as in the node 510and turns the VOA 214 in the fiber ring 202 back to its normalattenuation value to allow for optical transmission. At this time, thedetector PD1 in the node 510 in the fiber ring 202 monitors the opticaltransmission from the node 550 and the detector PD1 in the node 550 inthe fiber ring 201 monitors the optical transmission from the node 510.In addition, the central node 530 continues to detect the loss of theoptical probe signal in both fiber rings 201 and 202 and thus the gateswitches 211 and 212 remain closed to allow for optical transmissionthrough the central node 530.

After the fiber cut is repaired and optical transmission in both fiberrings between the nodes 510 and 550 is restored, the detector PD1 in thenode 510 in the fiber ring 202 detects light received from the node 550and the detector PD1 in the node 550 in the fiber ring 201 detects lightreceived from the node 510. This is shown in FIG. 6C. In response, thenodes 510 and 550 close their local switches by turning on the opticalamplifier 217 in the node 510 in the fiber ring 202 and the amplifier217 in the node 550 in the fiber ring 201 to return to their normaloperation.

After the above operations of the nodes 510 and 550, the circulatingoptical probe signals in both fiber rings 201 and 202 resume circulationin the fiber rings 201 and 202 through the nodes 510 and 550 so that theprobe detectors PD1 in the central node 530 detect the reappearance ofthe optical probe signals. The central node 530 responds to thiscondition by opening up the gate switches 211 and 212 to restore thedefault break points in the fiber rings 201 and 202, respectively. Thisis shown in FIG. 6D. At this time, the ring network 500 returns to itsnormal operation shown in FIG. 5.

In the above operations of the protection switching in the hybrid mode,the optical amplifiers 217 in nodes 510 and 550 may turn on at differenttimes when each node operates entirely based on its local detection ofthe total received optical power. This can occur, for example, when thefiber cuts in the two fiber rings at the same location are repaired atdifferent times. When the optical amplifiers 217 in nodes 510 and 550near the fiber break turn on at different times, the circulating opticalprobe signals in the two fiber rings 201 and 202 resume circulation inthe two fiber rings 201 and 202 at different times. The gate switches inthe central node 530 can be operated to open at different times whereeach gate switch in one fiber ring is closed as soon as the probedetector PD1 in the central node 530 in that fiber ring detects theoptical probe signal in order to avoid formation of a closed opticalloop in each fiber ring.

In deployment of many ring networks, it is often undesirable to turn onthe shut-down optical amplifiers 217 in nodes 510 and 550 at differenttimes in protection switching. TO address this technical issue, acentral control mechanism can be used to synchronize the operations ofturning on the shut-down optical amplifiers 217 in nodes 510 and 550during the protection switching. This can be achieved by, e.g., usingthe node-to-node OSC signaling or other management network signaling toturn on the shut-down amplifiers 217 at the same time like in thecentralized protection switching described by the example in FIGS.6A-6D. Therefore, after the amplifiers 217 in the nodes 510 and 550 areshut down, the operation of turning on the amplifier is controlled bythe central node 530.

First, after the fiber cut occurs, the detected signals by the localdetectors PD1 in the two neighbor nodes 510 and 550 next to the fibercut can indicate which of the two fibers has the fiber cut and whether asingle fiber or the two fibers experience a fiber cut. After the fibercut, the central node 530 opens up the two gate switches and the nodes510 and 550 shut down their local amplifiers 217 next to the fiber cutas described above and illustrated in FIG. 6B. The two nodes 510 and 550use the node-to-node signaling to inform the central node 530 of thestatus of the fiber cut condition. Next, the fiber cut is repaired andthe downstream optical detector PD1 can detect the optical transmissionthrough the repaired fiber span and this information is communicated tothe central node 530 via the node-to-node communications such as OSCsignaling. After the central node 530 is informed of that the singlefiber cut in one of the two fibers or the fiber cuts in both fibers arerepaired, the central node 530 sends out a command to each of the twonodes 510 and 550 via the node-to-node communication to turn on theshut-down optical amplifiers 217 in the nodes 510 and 550. Upon turningthe amplifiers 217, the optical probe signal reappears in both fiberrings in the central node 530 and, in response, the central node 530opens up the two gate switches 211 and 212 to restore the default breakpoints. At this time, the ring network automatically restores its normaloperation.

When a fiber cut occurs in only one fiber ring in the network 500 inFIG. 5, the same protection switching sequence shown in FIGS. 6A-6D canbe applied. Assuming the single fiber cut is in the fiber ring 202between the nodes 510 and 550, the fiber cut would first cause the node510 to shut down the optical transmission through the optical amplifier217 in the fiber ring 202 and the VOA 214 in the fiber ring 201. Unlessthe VOA 214 in the fiber ring 201 in the node 510 shuts down, the node550 would not detect the fiber cut and would continue its normaloperations. Once the VOA 214 in the fiber ring 201 in the node 510 shutsdown, the optical detector PD1 in the node 550 downstream from the node510 in the fiber ring 201 detects the loss of the optical power and thenode 550 shuts down the optical transmission of the optical amplifier217 in the fiber ring 201 and the VOA 214 in the fiber ring 202 withinthe node 550. The rest of the operations for the protective switchingcan be implemented based on the above hybrid mode protection switchingfor the fiber cuts in both fibers.

The special node 520 next to the central node 530 in FIG. 5 on the sideof gate switches 211 and 212 does not receive WDM optical signals fromthe central node and thus can use a node-to-node signaling with thecentral node 530 to determine whether there is a fiber break between thenode 520 and the central node 530 in the hybrid mode protectionswitching. In one implementation, this node-to-node signaling can be theOSC signaling as shown in the examples in FIGS. 7 and 8.

FIGS. 7 and 8 illustrate operations of the special node 520 based on OSCsignaling between the special node 520 and the central node 530 in FIG.5 according to one implementation. FIG. 7 shows a situation where afiber cut occurs in the fiber 201 between the nodes 520 and 530. Theprobe detector in fiber 201 within the central node 530 detects the lossof the circulating probe signal after the fiber cut occurs while thecirculating probe signal in the other fiber ring 202 remains unaffected.In response to this detected loss of the optical probe signal, thecentral node 530 closes both gates switches 211 and 212 in the fiberrings 201 and 202 to allow transmission through the central node 530 inboth fiber rings 201 and 202. At the same time, the OSC signal from thecentral node 530 to the special node 520 is detected by the special node520 as being missing. This OSC detection causes the special node 520 toopen local switches, VOA 214 and optical amplifier 217 close to thefiber cut, in both fiber rings 201 and 202 in the special node 520, toblock optical transmission through the special node 520.

After the fiber cut in the fiber ring 201 is repaired, the special node520 detects the OSC signal from the central node 530 again. The centralnode 530 can also inform the special node 520 through the OSC signalingfrom the central node 530 that the fiber span on fiber 202 and betweenthe central and special node is normal without a fiber break. Inresponse to the presence of the OSC signal from the central node 530,the special node 520 closes both switches VOA 214 and the amplifier 217in both fibers 201 and 202 close to the central node 530 to allowoptical transmission through the special node 520 in both fiber rings201 and 202. When this occurs, the central node 530 detects thecirculating optical probe signal in both fiber rings 201 and 202 andthus opens up the gate switches 211 and 212 to restore the default breakpoints in the fiber rings 201 and 202 to block optical transmissionthrough the central node 530. This restore the normal operatingcondition of the network 500 as shown in FIG. 5.

FIG. 8 further shows a network condition where a fiber cut occurs in thefiber ring 202 between the special node 520 and the central node 530.The central node 530 detects (1) the loss of the OSC signal from thespecial node 530 and (2) the loss of the optical probe signal in thefiber ring 202. The OSC signal from the central node 530 to the specialnode 520 in the fiber ring 201 is not affected by the fiber cut. Inresponse to the detected loss of the optical probe signal in the fiber202, the central node 530 closes gate switches 211 and 212 to allowoptical transmission through the central node 530. The central node 530also informs the special node 520 via the OSC signal in the fiber ring201 of the loss of the OSC signaling in the fiber ring 202. The specialnode 520 responds to this loss of the OSC signaling in the fiber ring202 by opening up both switches, the optical amplifier 217 and the VOA214 in the fiber rings 201 and 202 to block optical transmission throughthe special node 520. The VOA 214 in the special node 520 can bereplaced by a 1×1 optical switch. After the fiber cut in the fiber ring202 is repaired, the system restores back to its default status as shownin FIG. 5.

When fiber is cut in both fiber rings 201 and 202 between the specialnode 520 and the central node 530, both nodes 520 and 530 detect loss ofOSC signaling. The central node 530 also loses the optical probe signalin the fiber 202. This condition triggers the central node 530 to closeboth gate switches 211 and 212 to allow for optical transmission throughthe central node 530. The loss of the OSC signaling from the centralnode 530 at the special node 520 causes the special node 520 to open upboth switches, the optical amplifier 217 and the VOA 214 in the fiberrings 201 and 202, to block optical transmission through the specialnode 520. When fibers are repaired, the special node 520 and the centralnode 530 can communicate via the OSC signaling and the special node 520receives notification in the OSC signaling from the central node 530that both fibers 201 and 202 are repaired and the special node 520subsequently closes its two switches. This permits the optical probesignal to pass through the special node 520 to reach the central node530 in both fiber rings 201 and 202. In response to this reappearance ofthe optical probe signals in both fiber rings 201 and 202, the centralnode 530 opens the gate switches 211 and 212 to restore the defaultbreak points. At this time, the ring network returns to its normaloperation as shown in FIG. 5.

The above hybrid mode protection switching for a fiber cut between nodescan be similarly applied to protection switching for a device failure ina node by treating the device failure as a fiber cut in a nearest fiberspan to the failed device. Referring to FIG. 5, the failure of theoptical amplifier 217 in the fiber ring 202 within the node 510 istreated as a fiber cut in the fiber ring 202 between the nodes 510 and550 and thus the amplifier 217 in the fiber ring 201 in the oppositedirection in the fiber ring 201 of the nearest node 550 is shut downalong with the gates switches 211 and 212 in the central node 530 in theprotection switching. This switching can be triggered by the localdetection by the detectors PD2 and PD3 in the same node 510. This cancause local activation of the protection switching without the controlby the central node 530. The operating condition caused by the switchingis identical to what is in FIG. 6B. As another example, the failure ofthe VOA 214 in the fiber ring 202 within the node 510 can be detected bythe detectors PD1, PD2 and PD3 in the downstream node 520 and is treatedas a fiber cut in the fiber ring 202 between the nodes 510 and 520 andthus the amplifier 217 in the fiber ring 201 in the opposite directionin the nearest node 510 is shut down along with the gates switches 211and 212 in the central node 530 in the protection switching.

The above hybrid protection switching uses local detection in each nodeto activate local protection switching in each node affected by a fibercut or device failure without relying on control by the central node530. The central node 530 closes up the gate switches 211 and 212 toclose the default optical break points in the fiber rings 201 and 202based on the locally detected loss of the optical probe signal in atleast one fiber ring and does so without relying on communication fromother nodes. The two neighboring nodes closest to the fiber cut orfailed device also activates their protection switching entirely basedon the locally detected loss of the total optical power in one of thenodes without relying on commands from the central node 530 andcommunication from other nodes. Therefore, the node-to-nodecommunication process and the command from the central node 530 in thecentralized protection switching can be eliminated in the activation ofthe hybrid protection switching. In this regard, the hybrid protectionswitching is faster than the centralized protection switching. Thesecond phase of restoring the network back to its normal operation afterthe fiber cut or device failure is corrected in the hybrid protectionswitching, however, is similar to the centralized protection switchingand uses node-to-node communication and commands from the central node530 to synchronize the timing of the automatic reversion in both fiberrings 201 and 202.

In some optical ring networks, a particular optical signal carried by anoptical WDM signal is directed to a particular node in the ring and isnot intended for other nodes. In such a situation, after the particularnode receives and drops the particular optical signal, the optical WDMsignal that carries the dropped particular optical signal may no longerbe used by other nodes in a broadcast-and-select network and cannot bereused to carry a different optical signal. This is because of thenature of a broadcast-and-select network. The drop coupler in a nodesplits a portion of all optical signals in the fiber line and thedropped light is filtered to select one or more signals for that nodewhile optical power of the dropped one or more signals still remains inthe fiber line and propagates along with other signals to broadcast toother nodes. Hence, an added signal via the same node must have awavelength different from the dropped signals and other signals alreadyin the fiber line. After a signal at a designated WDM wavelength isdropped at a node and the signal is not used by other nodes, thedesignated WDM wavelength is wasted.

One way to reuse this wasted optical WDM wavelength is to implement awavelength blocker in each node to selectively block such an optical WDMsignal while allowing other WDM signals to transmit so that thecorrespond WDM wavelength can be reused again. In the above dual-fiberring networks in the centralized and hybrid control modes, each node canbe configured to place the drop optical coupler upstream to the addoptical coupler in each fiber so that a wavelength blocker can beinserted in each fiber ring downstream from the drop optical coupler andupstream from the add coupler.

FIG. 9 shows an example of a portion of a ring node associated withadding and dropping WDM signals in one fiber. For a node in a dual-fiberring network in FIG. 2 or 5, a similar mechanism can be duplicated forthe other fiber. As illustrated, the ring node 901 includes a dropoptical coupler 921 coupled to fiber 902 carrying optical traffic goingfrom the left to the right hand. The drop optical coupler 921 is abroadband optical coupler that splits a portion of optical power at alloptical WDM signals while allowing the remainder of all optical WDMsignal wavelengths to continue on in the fiber 902. An add coupler 931is coupled to the fiber 902 at a location downstream from the dropcoupler 921 to add one or more optical WDM signals to the fiber 902 forcommunications to other downstream ring nodes. A node receiver 920 isconnected to the drop coupler 921 to receive all WDM signals and selectsone or more WDM signals as dropped signals to extract information. Oneor more fixed optical bandpass filters or tunable optical bandpassfilers may be used for selecting the one or more dropped WDM opticalsignals. A node transmitter 930 is connected to the add coupler 931 toinclude one or more fixed or tunable optical transmitters to generatethe added WDM optical signals.

The node transmitter 930 in the node 901 in FIG. 9 can be implemented bya tunable transmitter in a network to achieve certain advantages.Consider an example where a network is updated by inserting a wavelengthblocker between the drop and add couplers in a node which has a nodetransmitter 930 at a WDM add wavelength different from a dropped WDMwavelength. If the node transmitter 930 in this node is tunable, thetransmitter wavelength produced by the tunable node transmitter 930 canbe immediately tuned to the dropped wavelength to re-use the droppedwavelength for transmitting a new signal channel. If a fixed wavelengthtransmitter were used as the node transmitters 930, this fixed nodetransmitter needs to be removed and replaced by a transmitter at thedropped wavelength after a wavelength blocker is inserted. Using atunable node transmitter can avoid the labor and cost associated withthe replacement

A wave blocker 910 is coupled in the fiber 902 between the drop coupler921 and the add coupler 931 to select at least one WDM wavelength tofilter out light at the selected one WDM wavelength while transmittingother WDM channels including the circulating optical probe signal.Accordingly, the node transmitter 930 can generate a new WDM signal atthe selected WDM wavelength to the fiber 902. In this context, theselected WDM wavelength that is filtered out and blocked by the waveblocker 910 can be reused by the ring network.

In the above fiber ring networks using a circulating optical probesignal, other fiber ring networks without the circulating optical probesignal and fiber networks in other network configurations, an opticalswitch may fail to open or close an optical break point. Such a switchfailure may affect operations of a portion or the entirety of a networkdepending on the location of the failed switch. In the fiber ringnetworks described in this application, the failure of one gate switch211 or 212 in the central node 210 in FIG. 2 or 530 in FIG. 5 may leadto failure of the protection switching for the entire network.Therefore, a redundant switch design may be desirable in implementingcertain optical switches to reduce the probability of complete failureof an optical switch.

FIG. 10A shows one example of a serial redundant switch pair as aswitching device 1001 where two optical switches 1021 and 1022 areconnected in series in an optical path 1010 such as a fiber line. Thisdevice 1001 can be used to provide redundancy in generating an opticalbreak point or opening between two terminals A and B in the optical path1010. If the optical switch 1021 fails to open, the second opticalswitch 1022 can be used to generate the desired optical break point inthe optical path 1010. This device 1001 can be used to protect a “hardto open” condition in an optical path. Three or more optical switchescan be used to further increase the redundancy of the device 1001.

FIG. 10B shows one example of a parallel redundant switch pair as aswitching device 1002 where two optical switches 1021 and 1022 areconnected in two parallel optical paths 1011 and 1012, respectively, inan optical path 1010 such as a fiber line. An optical splitter 1031 iscoupled in the optical path 1010 to connect to the first and secondoptical paths 1011 and 1012 and an optical combiner 1032 is coupled inthe optical path 1010 to join the first and second optical paths 1011and 1012 into the optical path 1010. This device 1002 provide twoalternative paths between two terminals A and B in the optical path 1010and to provide redundancy in connecting the two terminals A and B in theoptical path 1010. If the optical switch 1021 in the first optical path1011 fails to close, the second optical switch 1022 in the secondoptical path 1012 can be used to optically connect the two terminals Aand B in the optical path 1010. This device 1002 can be used to protecta “hard to close” condition in an optical path and can include three ormore parallel optical paths to further increase the redundancy of thedevice.

FIGS. 11A and 11B show two operation modes of a switching device 1100that combines the serial redundancy in FIG. 10A and the parallelredundancy in FIG. 10B. The first optical path 1012 includes two opticalswitches 1101 and 1102 connected in series and the second optical path1012 includes two optical switches 1021 and 1022 connected in series.This switching device 1100 can be used to protect both “hard to open”and “hard to close” conditions.

FIG. 11A shows an open mode of the switching device 1100 where all fouroptical switches 1101, 1102, 1021 and 1022 are open to opticallyseparate the two terminals A and B in the optical path 1010. To createan optical break point between the terminals A and B, each optical pathneeds to have one optical switch open. Another open mode for theswitching device 1100 can be switches 1101 and 1021 in their openpositions while switches 1102 and 1022 in their closed positions. Ifeither of the default open switches 1101 and 1021 fails to open, anotherswitch in the same path with the failed switch can be opened right away.

FIG. 11B shows a close mode of the device 1100 where the two switches1021 and 1022 in the first optical path 1011 are open to create anoptical break point between the two terminals A and B and the twoswitches 1101 and 1102 in the second optical path 1012 are closed toconnect the two terminals A and B. Here, the second optical path 1012 isthe default closed path to connect the terminals A and B. When one ofthe switches 1101 and 1102 fails to close, the switches 1021 and 1022 inthe first optical path 1011 are closed to form a closed path betweenterminals A and B.

The switching device 1100 can be used as an optical switch at anylocation where an optical switch is needed to provide reliable switchingoperations. The device 1100 has two 1×2 optical couplers 1031 and 1032and thus has approximately 7 dB optical loss. Using such a device inevery node in a switched ring architecture network to provide opticalswitching in each node would incur too much optical loss in the network.In the ring networks shown in FIGS. 2 and 5 where an optical switch isused in the central node while optical switching in other nodes iseffectuated by using optical amplifiers, the device 1100 can be used inthe central node to provide reliable optical switching in one of thefiber rings or both fiber rings.

In the above optical ring networks in the broadcast-and-selectarchitecture, optical WDM signals carrying traffic in each of the twoopposing ring propagation directions are directed and broadcast to allnodes in each ring network. These optical WDM signals are “in band”signals because their optical wavelengths are within the gain spectralrange of the optical amplifiers used in the ring network. Eachbroadcast-and-select optical WDM signal is dedicated to delivering dataand information from an originating node to one or more target nodes onthe ring and propagates through all locations on the ring except for anoptical break point or the protection gate switch in the central node.The optical WDM wavelength for each optical WDM signal originated fromone optical node is generally not shared with another optical node forsending data and information to the ring with exceptions. One exceptionis that when a particular optical WDM wavelength is blocked andterminated by a wavelength blocker at an optical node, the same opticalWDM wavelength can then be re-used by that optical node to launch a newoptical signal to the ring. FIG. 9 illustrates an example of thisexception.

Therefore, the bandwidth for carrying data and information traffic ineach broadcast-and-select optical WDM signal is dedicated to only oneoriginating optical node for sending the data and information to one ormore selected destination optical nodes or all other optical nodes onthe ring. The bandwidth of this optical WDM signal is not shared withother optical nodes. One of advantages of this dedicated bandwidth foreach optical WDM signal is that the bandwidth is guaranteed for theoptical node that is assigned to use that optical WDM wavelength to sendout data and information to the ring.

FIG. 12 illustrates logical communications for nodes in an example of abroadcast-and-select ring using dedicated broadcast-and-select opticalWDM signals. Optical nodes are labeled by letters A, C, D, E, F, G, H,I, J, K and L. The optical node A is the optical gate switch node thatprovides the optical protection switching to maintain a single opticalbreak point within the ring during a normal operation and when there isa single optical break point. Referring to the example in FIG. 2, thecentral node 210 is an example of the node A in FIG. 12. Multiplededicated broadcast-and-select optical WDM wavelengths are used to carryoptical traffic in the ring in FIG. 12. Eight dashed lines are used torepresent communications between nodes. For example, a first optical WDMwavelength is dedicated to provide communication from node C to nodes Jand K; a second optical WDM wavelength is dedicated to providecommunication from node A to nodes D, F and G; and a third optical WDMwavelength is dedicated to provide communication from node D to nodes Aand I. Alternatively, the communications from node A to nodes D, F and Gmay be three different signals carried by three different dedicatedoptical WDM wavelengths.

In some applications, the available bandwidth of an optical WDMwavelength may not be fully used by the associated optical node. Underthe above ring network designs, the unused portion of the bandwidthcannot be shared or used by other nodes on the ring and thus is wasted.The following sections describe examples of ring networks that, inaddition to the above described dedicated in-band broadcast-and-selectoptical WDM wavelengths, allocate one or more optical WDM wavelengthsfor carrying data and information traffic through node-to-nodecommunications with the optical-electrical-optical conversion withineach node to allow different nodes to share an optical WDM wavelength.Different from a dedicated broadcast-and-select optical wavelength whichcirculates around the ring through all nodes in absence of an opticalfailure and a fraction of which is split at each node, such anode-to-node optical signal at a shared optical WDM wavelength isgenerated by an optical transmitter at one node and is opticallyterminated at the next node along one propagation direction of theoptical ring. The receiving node regenerates the node-to-nod opticalsignal at the shared wavelength using an optical transmitter forpropagation to yet another node downstream along the propagationdirection. Hence, at each node, the node-to-nod optical signal at theshared wavelength undergoes an optical-to-electrical conversion at anoptical receiver and then an electrical-to-optical conversion for theregeneration by an optical transmitter. The dropping and adding data areaccomplished in the electronic domain in each node. In another aspect,each node-to-node optical WDM signal is different from a dedicatedbroadcast-and-select optical wavelength which carries only data andinformation originated at a single node in that each node-to-nodeoptical WDM signal can carry data and information originated bydifferent optical nodes. Each node can add new data to the node-to-nodeoptical WDM signal in the electrical-to-optical conversion at the node'soptical transmitter.

Such a node-to-node optical WDM signal shared by different nodes can beat an “in band” optical WDM wavelength within the gain spectral range ofthe optical amplifiers used in the ring network. Alternatively, becausesuch a node-to-node optical WDM signal only travels through a fiber spanbetween two adjacent nodes before being regenerated, an “out of band”optical WDM wavelength at the edge or outside the gain spectral range ofthe optical amplifiers used in the ring network can be used in some ringnetworks where the fiber span between two adjacent nodes is sufficientlyshort so that optical amplification of the node-to-node optical WDMsignal is not necessary. The examples described below use in bandoptical WDM wavelengths for node-to-node optical WDM signals and variousfeatures in these examples are applicable to ring networks that useout-of-band optical WDM wavelengths for node-to-node optical WDMsignals.

Each node-to-node optical WDM signal is an optical signal thatpropagates along with the dedicated broadcast-and-select optical WDMwavelengths and thus is overlaid with the dedicated broadcast-and-selectoptical WDM wavelengths in the same fiber. To provide communicationredundancy, each node-to-node optical WDM signal can be duplicated ineach of the two counter propagating directions of a ring network.Notably, this use of dedicated broadcast-and-select optical WDMwavelengths and one or more node-to-node optical WDM wavelengths forcarrying communication traffic in a ring can be implemented in not onlyring networks with the circulating optical probe signal for protectionswitching described above but also other ring network designs havingcounter-propagating traffic signals.

The overlay of broadcast-and-select dedicated bandwidth and node-to-nodeshared bandwidth on the same two fibers in ring networks can beimplemented to achieve one or more advantages such as efficient use ofthe available optical bandwidth and reduction in unused bandwidth,improved communication speed and quality of service. For example, a ringnetwork may have a mixture of heavy and light nodes so that differentnodes have different traffic loads. If only broadcast-and-selectdedicated bandwidths are used, a broadcast-and-select dedicatedbandwidth assigned to a light node may not be fully used and the unusedportion is wasted. Under the overlay of the broadcast-and-selectdedicated bandwidth and node-to-node shared bandwidth, heavy nodes canbe served by provisioning them with signals that have a dedicatedbandwidth for each heavy node, while light nodes can be served byprovisioning them with one or more node-to-node optical WDM signals witha shared bandwidth. A node may also be served by both abroadcast-and-select optical WDM signal with a dedicated bandwidth andone or more node-to-node optical WDM signals with a shared bandwidth.

The allocation or provisioning of the broadcast-and-select dedicatedbandwidth and the node-to-node shared bandwidth can be determined basedon one or more parameters associated with each node, such as nodetraffic measured by a node traffic parameter (e.g., the number of bitsper second at a node), the level of the quality of service (QoS) andlatency requirement at each node, or a combination of the node traffic,the QoS, and latency at each node. For example, a node with a service ata high level of QoS and low latency requirement may be allocated with abroadcast-and-select optical WDM signal with a dedicated bandwidth evenwhen the traffic at the node is not heavy.

The allocation or provisioning of the broadcast-and-select dedicatedbandwidth and the node-to-node shared bandwidth may be fixed based onthe initial allocation or adjusted after the initial allocation inresponse to changes to the ring networks. For example, a node that isinitially allocated with a broadcast-and-select optical WDM signal andis not initially allocated to use a node-to-node optical WDM signal canbe later allocated to share the bandwidth of a node-to-node optical WDMsignal so that this node can transmit with both the shared node-to-nodeoptical WDM signal and the dedicated broadcast-and-select optical WDMsignal.

FIG. 13 illustrates a ring network that adds one or more in-bandnode-to-node optical WDM signals to the optical ring shown in FIG. 12.The added one or more in-band node-to-node optical WDM signals arerepresented by solid curve lines connecting adjacent nodes. A singlenode-to-node optical WDM signal at a given WDM wavelength dedicated fornode-to-node communications can be used to carry data originated fromone or more nodes so that the bandwidth at this given WDM wavelength canbe shared by multiple nodes. A node-to-node optical WDM signal along onering direction is generated at one node and is terminated at theadjacent neighbor node. A new node-to-node signal is generated at theadjacent neighbor node to propagate to the next adjacent neighbor node.This process of signal generation, termination and regeneration repeatsthrough the nodes along each ring direction until the destination nodeis reached. As an example, data generated at the node A for the node Gis carried by different node-to-node signals at the same designated WDMwavelength from node A to node C, from node C to node D, from node D tonode E, from node E to node F, and from node F to node G along one ringdirection and, from node A to node L, from node L to node K, from node Kto node J, from node J to node I, from node Ito node H, and from node Hto node G along the other ring direction. The wavelengths reserved forcarrying node-to-node traffic are uniform throughout the entire ringnetwork.

In implementations where the node-to-node optical WDM signals use thein-band WDM wavelengths, the bandwidth of the available WDM channelwavelengths can be divided between the broadcast-and-select traffic andthe node-to-node traffic where a majority of WDM wavelengths isdesignated for the broadcast-and-select traffic and a fraction of theavailable WDM wavelengths is used for the node-to-node traffic. Forexample, a majority (e.g., 80%) of the C-band bandwidth can be allocatedto dedicated broadcast-and-select optical WDM wavelengths while theremaining of the C-band bandwidth is allocated to shared node-to-nodeoptical WDM wavelengths. Hence, if there are 40 in-band WDM wavelengthsavailable in the C-band, 32 WDM wavelengths can be used for dedicatedbroadcast-and-select optical WDM wavelengths and 8 WDM wavelengths canbe used for shared node-to-node optical WDM wavelengths. The 8 sharednode-to-node optical WDM wavelengths can be repeatedly used in everyfiber span in the optical ring to be reused in every fiber span. The 32dedicated broadcast-and-select optical WDM wavelengths are not reusedexcept in nodes where wavelength blockers orwavelength-selective-switches are inserted between the drop and addports.

FIG. 14 illustrates an exemplary dual-fiber ring network 1400 thatimplements dedicated broadcast-and-select optical WDM wavelengths andone or more in-band node-to-node optical WDM wavelengths for carryingcommunication traffic. Each optical node can be configured to providebroadcast-and-select functions in connection with the dedicatedbroadcast-and-select optical WDM wavelengths and to support in-bandnode-to-node optical WDM wavelengths. As an example, each optical nodein FIG. 14 can include an optical add/drop module 1401 for handlingdedicated broadcast-and-select optical WDM wavelengths and opticaladd/drop modules 1440 and 1430 for handling the in-band node-to-nodeoptical WDM wavelengths.

In the illustrated example, the optical add/drop module 1401 includes atleast one optical transmitter 1410 to produce an optical add signal atan optical WDM wavelength. An optical splitter 1412 splits the opticaladd signal into two optical add signals to be added in two oppositedirections in the ring. A first add coupler 1414 is coupled to the firstfiber ring path 201 to add one or more optical add signals to the firstring path 201 along a first ring direction which is the counterclockwise direction. Similarly, a second add coupler 1416 is coupled tothe second ring path 202 to add the one or more optical add signals tothe second ring path 202 along the second ring direction, e.g., theclockwise direction. An optical receiver 1420 is provided to receive thedropped optical signal at the node 1400. A first drop coupler 1426 iscoupled to the first ring path 201 to drop the optical signals from thefirst ring path 201 along the first ring direction and a second dropcoupler 1424 is coupled to the second ring path 202 to drop the opticalsignals from the second ring path 202 along the second ring direction.The dropped signals are directed to the optical receiver 1420 fordetection.

The optical add/drop modules 1440 and 1430 for handling the in-bandnode-to-node optical WDM wavelengths can include wavelength-selectiveoptical couplers to selectively add or drop the one or more in in-bandnode-to-node optical WDM wavelengths and to allow the dedicatedbroadcast-and-select optical WDM wavelengths and other wavelengths topass through. The optical add/drop module 1440 is located at theleft-hand side of the optical node to include a wavelength-selectiveoptical add coupler 1441 to add one or more in-band node-to-node opticalWDM wavelengths along a direction to a downstream optical node in onefiber ring path 201 and to include a wavelength-selective optical dropcoupler 1442 to drop and terminate one or more in-band node-to-nodeoptical WDM wavelengths along the opposite direction received from anupstream optical node in the other fiber 202. The optical add/dropmodule 1430 is located at the right-hand side of the optical node issimilarly constructed with a wavelength-selective add coupler 1432 and awavelength-selective drop coupler 1431.

FIG. 15 shows an exemplary implementation of the generation anddetection of the in-band node-to-node optical WDM signals. In thisexample, an optical receiver 1512 (RX1) and optical transmitter 1511(TX1) are coupled to the module 1430 to use the optical receiver 1512 toconvert the received in-band node-to-node optical WDM signals intoelectrical signals and to use the optical transmitter 1511 to producethe in-band node-to-node optical WDM signals going to the downstreamalong the fiber ring path 202. Similarly for the module 1440 on theother side of the node, an optical receiver 1522 (RX2) and opticaltransmitter 1521 (TX2) are coupled to the module 1440 to use the opticalreceiver 1522 to convert the received in-band node-to-node optical WDMsignals into electrical signals and to use the optical transmitter 1521to produce the in-band node-to-node optical WDM signals going to thedownstream along the fiber ring path 201. A node-to-node electroniccontrol module 1500 is provided to perform signal detection, signaldropping, signal adding, signal generation and signal switching in theelectrical domain. Adding signals originated at different optical nodesto a single in-band node-to-node optical WDM signal can be achieved bythe control module 1500. New in-band node-to-node optical WDMwavelengths are generated by the optical transmitters 1511 and 1521under the control of the control module 1500 and are added to the twofibers 201 and 202 to propagate to their respective downstream opticalnodes.

The control module 1500 can be implemented in various configurationsdepending on specific requirements in applications. For example, thecontrol module 1500 may include a node-to-node controller thatgenerates, controls and processes the electrical signals that drive theoptical transmitters TX1 (1511) and TX2 (1521) and electrical signalsfrom RX1 (1512) and RX2 (1522) based on the dropped in-band node-to-nodeoptical signals. For another example, the control module 1500 mayinclude an electronic signal router or switch for routing electricalsignals that drive the optical transmitters TX1 (1511) and TX2 (1521)and electrical signals from RX1 (1512) and RX2 (1522) based on thedropped in-band node-to-node optical signals. For yet another example,the control module 1500 may include an SONET/SDH add/drop multiplexer(ADM) device that controls the signals associated with opticaltransmitters TX1 (1511) and TX2 (1521) and optical receivers RX1 (1512)and RX2 (1522).

The protection scheme for the traffic carried by node-to-node opticalWDM wavelengths may be implemented by electronic switching via the layer2 or layer 3 routing of a GbE/10 GbE/100 GbE switch/router built into orin communication with the OEO module 1500. Examples of layer 2/3protection schemes include spanning-tree-protocols,rapid-spanning-tree-protocols, and MPLS (multi-protocol label switching)schemes. In other implementations, SONET/SDH protection scheme (UPSR,BLSR) for SONET/SDH equipment may also be used to provide protectionscheme for the traffic carried by in-band node-to-node optical WDMwavelengths.

FIG. 16 shows another example node design for dropping and adding onenode-to-node WDM signal at one WDM wavelength in each of the two fiberswhere an optical switch 1611, e.g., a 1×2 switch, is used to receive twodropped node-to-node optical WDM signals respectively from the twofibers 201 and 202 and to direct one of the two dropped node-to-nodeoptical WDM signals into an optical receiver within an opticaltransponder 1620. The optical receiver produces an electronic signal1621 for detection and processing by a node-to-node electronic controlmodule 1630. The optical switch 1611 is set at a default switching stateto select one dropped node-to-node optical WDM signal (e.g., the signaldropped from the fiber 202) as the default signal to the opticaltransponder 1620. When this default signal is detected to be missing orits signal level is below a pre-set threshold, the optical switch 1611switches to direct the other dropped node-to-node optical WDM signal(e.g., the dropped signal from the fiber 201) to the optical transponder1620. The optical receiver in the optical transponder 1620 may be usedto monitor the status of the default drop signal and to control theoperation of the switch 1611. A separate optical detector may be used ineach node to detect each dropped signal for controlling the switch 1611.The control module 1630 produces an add electric signal 1622 to controlan optical transmitter in the optical transponder 1620 to produce an addnode-to-node optical WDM signal. An optical coupler 1612, e.g., a 1×2coupler, is used to receive this add node-to-node optical WDM signal andto splits the received add node-to-node optical WDM signal into two addnode-to-node optical WDM signals that are directed to the two couplers1411 and 1432, respectively, for adding to the two fibers 201 and 202 inopposite ring directions. In this design, an optical transponder with asingle optical receiver and a single optical transmitter can be used asthe optical transponder 1620. The control module 1630 can be configuredto produce various functions in the electronic domain similar to thoseby the control module 1500 in FIG. 15.

The protection switching for the in-band dedicated broadcast-and-selectoptical WDM wavelengths is separated from the protection scheme for thenode-to-node optical WDM wavelengths and can be implemented by variousprotection switching configurations. For example, the opticalsupervision channel (OSC) mechanism can be used to provide node-to-nodecommunication channel for reporting an optical failure in the fiber ringand to control the protection switching. The optical protectionswitching techniques based on the circulating optical probe signaldescribed in this application can also be used to implement theprotection switching for the dedicated broadcast-and-select optical WDMwavelengths.

Referring back to FIGS. 13 through 16, the combination of thenode-to-node optical WDM signals and the in-band broadcast-and-selectoptical WDM signals makes each fiber span between two adjacent nodesoptically active to carry communication traffic in both ring directionsfor the ring network. This aspect of the ring networks in FIGS. 13through 16 is different from the hybrid ring network in FIG. 5 where thefiber span between the central node with the protection switches and oneof the neighboring nodes of the central node does not carrycommunication traffic in at least one of the ring directions during thenormal operation. In another aspect, the presence of in-bandnode-to-node optical signals in FIG. 14, in absence of the circulatingprobe signal, can make it difficult to detect a fiber break due toleaking of the node-to-node signals into a regular node because theadd/drop couplers 1431 and 1442 may be imperfect in actualimplementations and may leak some of the light in the in-bandnode-to-node signals to a respective optical power detector that detectsthe total optical power of the received light at the optical powerdetector. Therefore, a fiber break or an amplifier failure locatedbefore the upstream added node-to-node signal might not be easilydetected if a simple optical power detector were used to detector thetotal received optical power. The use of the circulating optical probein the ring and the probe detector in each node for selectivelydetecting light at the circulating probe wavelength can be implementedin combination with the in-band node-to-node communication signals toenhance the detectability of a fiber break or an amplifier failure underthe overlaid in-band node-to-node traffic designs of ring networks.

The above use of node-to-node communication signals for carrying trafficfor ring networks can be expanded to optical wavelengths that are either(1) out of the spectral range of the optical amplifiers in the ringnetworks or (2) at the edge of the spectral range of the opticalamplifiers. This is because each node-to-node signal carrying trafficdata only travels through a fiber span between two adjacent nodes and isregenerated at each node. The optical loss in each optical fiber spanbetween adjacent nodes can be small and does not affect thedetectability of the signal. The designs and operations of the nodedesigns for the in-band node-to-node signals for carrying traffic can beapplied to ring networks that use out-of-band node-to-node signals tocarry traffic from one node to the next to allow communicationthroughout the entire ring.

The implementations and examples described above can use the loss of theOSC signal to determine presence of an optical break in the fiber spanbetween two adjacent nodes for the protection switching, with or withoutthe overlaid in-band node-to-node traffic designs. The loss of the OSCsignal in a particular span may be caused by either (1) an actualoptical break or failure in the fiber span or (2) a failure in an OSCcard in either the upstream node or the downstream node along one of thetwo ring directions. Hence, when the condition (2) occurs, the loss ofthe OSC signal cannot be used to trigger the protection switching. Onemethod to prevent the condition (2) from triggering the protectionswitching is to use presence of the circulating probe signal at thecentral node as an indicator whether there is an actual optical break inthe fiber span where the respective OSC signal is lost. When an OSCsignal is used for monitoring a break in a fiber span and for triggeringthe protection switching, the protection switching control mechanism inthe ring network can be configured to require a confirmation signalbroadcast from the central node to all nodes that the probe wavelengthis lost in the central node. Only after this confirmation signal isreceived after the loss of the OSC occurs in a fiber span, the localoptical switches or amplifiers in the fiber span can be operated tocreate a protective optical break. This is an example of using both theOSC signal and the circulating optical probe signal for protectionswitching.

FIG. 17 shows an example of a ring network that has regular opticalnodes and a central node 1710 equipped with multi-channel DWDMmultiplexers, multi-channel DWDM demultiplexers, and a centraloptical-to-electrical-to-optical (OEO) conversion block, where bothdedicated broadcast-and-select optical WDM signals and overlaid in-bandnode-to-node optical WDM signals are used for carrying optical traffic.No circulating optical probe light is used for protection switching inthis design. Instead, the node-to-node OSC signal is used for protectionswitching. The fiber protection switching in this ring network canimplement a tail-end switching in connection with the OEO conversionblock in the central node. This and other optical ring networksdescribed in this application may be used in various communicationsystems, e.g., access networks, backbone networks, and other networks.Cable television systems, video-on-demand delivery systems, and othercommunication service systems may use fiber rings described here. Anoutput optical signal from a ring node may be a broadcast signal to allnodes in the ring, a multicast signal to selected nodes in the ring, ora signal to a selected single node. Optical signals at differentwavelengths may be divided into bands for purpose of communicationmanagement, where each band may include one or more optical signals atdifferent wavelengths. These bands and ITU channels may be dropped oradded at each node. In addition, multiple wavelengths for differentchannels may be closely packed within each ITU grid to increase thenumber of WDM channels beyond the common arrangement of one channel perITU grid. Accordingly, high resolution tunable or fixed narrow passbandoptical filters or WDM demultiplexers may be used to separate theclosely spaced channel wavelengths within each ITU grid. One way togenerate such closely spaced wavelengths within each ITU grid, as anexample, is to use subcarrier multiplexing by interleavingsubcarrier-sidebands and suppressing optical carrier from multipleseparately modulated subcarriers.

Each regular optical node in FIG. 17 can be constructed to allow forselective dropping one or more optical WDM channels while allowingtransmission of some or all optical WDM channels through the nodewithout optical-to-electrical-to-optical conversion as in the centralnode 1710. One example for the regular optical node design is shown inFIG. 2, such as the nodes 220 (node 2) and 230 (node 3). The centralnode 1710, in one aspect, is similar to other nodes in the ring andincludes optical add/drop modules 1440 and 1430 for handling thenode-to-node optical WDM wavelengths. Different from other nodes in thering, the central node 1710 includes multi-channel DWDM multiplexers1712 and 1722, multi-channel DWDM demultiplexers 1711 and 1721, andcentral optical-to-electrical-to-optical (OEO) conversion blocks 1731and 1732 for handling the dedicated in-band broadcast-and-select opticalWDM wavelengths by regenerating all dedicated in-bandbroadcast-and-select optical WDM wavelength. In contrast, other opticalnodes generate only one or more pre-assigned WDM wavelengths whileallowing optical transmission of other WDM wavelengths withoutregeneration.

The demultiplexer 1711 separates received light in the fiber 201 intomultiple WDM signals at different WDM wavelengths along separate opticalpaths, respectively. The OEO block 1731 converts the WDM signals intoelectrical signals and process the electrical signals to produce newoptical WDM signals that are combined by the multiplexer 1712 into theoutput side of the node 1710 on the fiber 201. Similarly, thedemultiplexer 1721 separates received light in the fiber 202 intomultiple WDM signals at different WDM wavelengths along separate opticalpaths, respectively. The OEO block 1732 converts the WDM signals intoelectrical signals and process the electrical signals to produce newoptical WDM signals that are combined by the multiplexer 1722 into theoutput side of the node 1710 on the fiber 202. This design does notrequire the operation of ring switches and probe wavelengths for opticalswitching, and the ring fiber protection relies on a tail-end switchingvia the OEO blocks 1731 and 1732, such as in an Optical Uni-directionalProtection Switch Ring (UPSR). The OEO blocks 1731 and 1732 provide adefault optical break point at all times in each ring to preventundesired optical oscillation. The ring network in FIG. 17 uses the OSCmechanism with OSC modules in each node for the protection switching andthe central node 1710 is responsive to a status of the OSC signal tocontrol a protection switching mechanism within the OEO block to sustaincommunications in the optical ring network when the OSC signal indicatesan optical failure.

The optical nodes in the above examples of ring networks with either oneor both of (1) the circulating optical probe signal in the ring and (2)the node-to-node communications can be configured to have pre-configuredadd and drop optical ports at pre-selected optical add and drop WDMwavelengths. Ring networks with such pre-configured add and dropoperations in the nodes can suffer various limitations. For example,such a network requires replacement of an optical node when thepre-configured add and drop operations at that optical node need to bechanged. This replacement process requires labor and adds cost to thenetwork operation and maintenance.

Alternatively, the optical nodes in the above examples of ring networkswith either one or both of (1) the circulating optical probe signal inthe ring and (2) the node-to-node communications can be implemented tohave optical node's based on reconfigurable optical add and dropmultiplexers or modules (ROADM) for various applications, such as metroand regional optical networks. An optical node based on a ROADM designcan be configured to allow flexible wavelength add/drop in a way similarto the flexible frame add/drop operations in a SONET/SDH add/drop moduleor multiplexer (ADM). Different from SONET/SDH ADM, an optical ROADM canbe configured to have a different add/drop granularity in wavelength andcan eliminate optical-electrical-optical regeneration in the SONET/SDHADM. ROADM-based ring networks with node-to-node communication channelscan be used to provide desired flexibility in network deployment toallow for adjustments in various aspects in an existing network.

FIG. 18 shows an example of an ROADM 1800 for reconfigurable add anddrop operations of optical WDM channels in one direction in a ringnetwork. This ROADM 1800 includes a drop module 1801 that drops one ormore selected optical WDM channels and transmits the remaining opticalWDM signals (express WDM wavelengths) and an add module 1820 thatreceives light from the drop module 1810 and adds one or more selectedoptical WDM channels at available WDM wavelengths. Each of the dropmodule 1810 and the add module 1820 is reconfigurable to control andadjust the number of add or drop WDM wavelengths and the wavelengths ofthe add or drop WDM wavelengths so that the ROADM 1800 can operate withonly one drop port for dropping one WDM wavelength and one add port foradding one add WDM wavelength and scale up to include additional add ordrop ports as needed. The wavelength of the circulating optical probesignal for the switching protection can be selected to be different fromthe optical WDM wavelengths that are allocated to the ROADMs in thenetwork and, as such, operations associated with detecting thecirculating optical probe signal are independent of the operations ofROADMs. Similarly, the wavelength of a node-to-node opticalcommunication channel can be selected to be different from the opticalWDM wavelengths that are allocated to the ROADMs in the network and, assuch, operations associated with the node-to-node communications areindependent of the operations of ROADMs.

As an example, the drop module 1810 can include an optical WDMdemultiplexer to separate the incoming WDM channels, an optical switcharray that receive the separated incoming WDM channels and switchindividual WDM channels to different optical ports for drop operationsand to transmit the express WDM channels as output WDM channels to theadd module 1820. Optical amplifiers or variable optical attenuators maybe included in optical paths of individual optical WDM signals withinthe drop module 1810 to control the optical signal strengths of theoptical WDM channels. Similarly, the add module 1820 can include anoptical switch array to direct one or more optical add WDM channels andthe express optical WDM channels from the drop module 1810 to an opticalWDM multiplexer to combine such optical WDM channels as outgoing WDMchannels to the next optical node in the ring network.

In ring networks with either one or both of (1) the circulating opticalprobe signal in the ring and (2) the node-to-node communicationsdescribed above, ROADMs in optical nodes can be designed and operated toadd or drop any of the optical WDM wavelengths for the ring network dueto the reconfigurability and are essentially free of node-specificrestrictions. Therefore, the same ROADMs can be placed in any networknode in the ring network. Notably, the number of add and drop opticalWDM wavelengths and the specific wavelengths of for the add and dropoperations at each node are re-configurable in response to a controlcommand. Therefore, reconfiguring an optical node with a ROADM can beachieved via software without any changes to the hardware of the opticalnode. Broadcast-and-select operations can be implemented via ROADMs forvarious applications such as multicast video-conference and broadcastdigital video applications.

FIG. 18 further shows that optical amplifiers with variable opticalgains or variable optical attenuators may be provided in optical pathsof optical WDM channels to control the signal strengths of the opticalWDM channels, e.g., equalizing the signal strengths. In the particularexample as shown, optical amplifiers or attenuators are placed in theseparated optical paths of different optical WDM channels between thedrop module 1810 and the add module 1820 and optical attenuators areplaced in the separated optical paths of different optical add WDMchannels. Optical detectors can be implemented to monitor optical powerlevels of different optical WDM channels and the measured optical powerlevels are used to control the optical amplifiers or opticalattenuators. Such power equalization and monitoring functions for eachof the express and add WDM wavelengths can improve the performance ofthe optical WDM ring network.

While this patent application contains many specifics, these should notbe construed as limitations on the scope of the invention or of what maybe claimed, but rather as descriptions of features specific toparticular embodiments of the invention. Certain features that aredescribed in this patent application in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Only a few implementations are disclosed. However, it is understood thatvariations and enhancements of the described implementations and otherimplementations can be made based on what is described and illustrated.

1. An optical communication system, comprising: a plurality of opticalring nodes connected to form an optical ring which support (1)broadcast-and-select optical WDM signals carrying communication trafficto the optical ring nodes without regeneration at each optical ringnode, and (2) at least one node-to-node optical WDM signal carryingcommunication traffic from one optical ring node to an adjacent opticalring node by regeneration at each optical ring node, wherein thenode-to-node optical WDM signal is at a wavelength different fromwavelengths of the broadcast-and-select optical WDM signals; and aplurality of optical amplifiers coupled in the optical ring and operableto amplify light in a gain spectral range covering optical wavelengthsof the broadcast-and-select optical WDM signals.
 2. The system as inclaim 1, comprising: an optical probe transmitter coupled to the opticalring network to supply to the optical ring network an optical probesignal at a probe wavelength which is inside or near one end of the gainspectral range to obtain a sufficient optical gain from the opticalamplifiers to sustain a detectable signal level; a probe monitor in atleast one optical ring node to split a portion of the optical probesignal and to monitor the optical probe signal to detect an opticalfailure in the optical ring network; and a protection switch in theoptical ring network to create a default optical break point when theprobe monitor indicates that there is no optical failure in the opticalring network and to close the default optical break point when the probemonitor indicates that there is an optical failure, wherein the opticalring network is responsive to a status of the monitored optical probesignal to control the protection switch and the optical ring nodes tosustain communications in the optical ring network outside a location ofthe optical failure and to restore communications in the optical ringnetwork after the optical failure is repaired.
 3. The system as in claim1, wherein the at least one node-to-node optical WDM signal is withinthe gain spectral range of the optical amplifiers.
 4. The system as inclaim 1, wherein the at least one node-to-node optical WDM signal isoutside the gain spectral range of the optical amplifiers.
 5. The systemas in claim 1, wherein the optical ring includes first and second fibersto connect two adjacent nodes to carry optical traffic in oppositedirections, respectively.
 6. The system as in claim 5, wherein each nodecomprises: an optical add and drop module coupled to the first andsecond fibers to produce one or more of the broadcast-and-select opticalWDM signals and to split a fraction of light in the broadcast-and-selectoptical WDM signals for detection while transmitting rest of the lightin the broadcast-and-select optical WDM signals; a first node-to-nodeoptical module that is coupled to the first and second fibers downstreamin the first fiber from the optical add and drop module and comprises: afirst node-to-node optical transmitter to produce the node-to-nodeoptical WDM signal for the first fiber to a downstream optical node inthe first fiber; a first optical add coupler coupled to the first fiberto direct the node-to-node optical WDM signal into the first fibertowards the downstream optical node in the first fiber; a first opticaldrop coupler coupled to the second fiber upstream to the optical add anddrop module to selectively couple light of the node-to-node optical WDMsignal from an upstream optical node in the second fiber out of thesecond fiber while transmitting light of other wavelengths including thebroadcast-and-select optical WDM signals; and a first optical receivercoupled to receive the node-to-node optical WDM signal from the firstoptical drop coupler; and a second node-to-node optical module that iscoupled to the first and second fibers upstream in the first fiber tothe optical add and drop module and comprises: a second node-to-nodeoptical transmitter to produce the node-to-node optical WDM signal forthe second fiber to a downstream optical node in the second fiber; asecond optical add coupler coupled to the second fiber to direct thenode-to-node optical WDM signal into the second fiber towards thedownstream optical node in the second fiber; a second optical dropcoupler coupled to the first fiber upstream to the optical add and dropmodule to selectively couple light of the node-to-node optical WDMsignal from an upstream optical node in the first fiber out of the firstfiber while transmitting light of other wavelengths including thebroadcast-and-select optical WDM signals; and a second optical receivercoupled to receive the node-to-node optical WDM signal from the secondoptical drop coupler.
 7. The system as in claim 6, wherein each nodecomprises an electronic switching protection mechanism to provideswitching for the node-to-node communication traffic.
 8. The system asin claim 6, comprising: an optical supervision channel (OSC) mechanismoperable to produce an OSC signal at an OSC wavelength between twoadjacent two optical ring nodes in each of the first and second fibersto communicate information on the ring network other than data traffic,the OSC mechanism comprising: in each optical ring node, a first OSCoptical module that is coupled to the first and second fibers downstreamin the first fiber from the optical add and drop module and comprises: afirst OSC optical transmitter to produce the OSC signal for the firstfiber to a downstream optical node in the first fiber; a first OSC addcoupler coupled to the first fiber to direct the OSC signal into thefirst fiber towards the downstream optical node in the first fiber; afirst OSC drop coupler coupled to the second fiber upstream to theoptical add and drop module to selectively couple light of the OSCoptical WDM signal from an upstream optical node in the second fiber outof the second fiber while transmitting light of other wavelengths; and afirst OSC receiver coupled to receive the OSC signal from the first OSCdrop coupler; and a second OSC optical module that is coupled to thefirst and second fibers upstream in the first fiber to the optical addand drop module and comprises: a second OSC transmitter to produce theOSC signal for the second fiber to a downstream optical node in thesecond fiber; a second OSC add coupler coupled to the second fiber todirect the OSC signal into the second fiber towards the downstreamoptical node in the second fiber; a second OSC drop coupler coupled tothe first fiber upstream to the optical add and drop module toselectively couple light of the OSC signal from an upstream optical nodein the first fiber out of the first fiber while transmitting light ofother wavelengths; and a second OSC receiver coupled to receive the OSCsignal from the second optical drop coupler.
 9. The system as in claim8, comprising: an optical probe transmitter coupled to the optical ringnetwork to supply to the optical ring network an optical probe signal ata probe wavelength which is inside or near one end of the gain spectralrange to obtain a sufficient optical gain from the optical amplifiers tosustain a detectable signal level; a probe monitor in at least oneoptical ring node to split a portion of the optical probe signal and tomonitor the optical probe signal to detect an optical failure in theoptical ring network; and a protection switch in the optical ringnetwork to create a default optical break point when the probe monitorindicates that there is no optical failure in the optical ring networkand to close the default optical break point when the probe monitorindicates that there is an optical failure, wherein the optical ringnetwork is responsive to both a status of the monitored optical probesignal and a status of the OSC signal to control the protection switchand the optical ring nodes to sustain communications in the optical ringnetwork outside a location of the optical failure and to restorecommunications in the optical ring network after the optical failure isrepaired.
 10. The system as in claim 5, wherein each node comprises: anoptical add and drop module coupled to the first and second fibers toproduce one or more of the broadcast-and-select optical WDM signals andto split a fraction of light in the broadcast-and-select optical WDMsignals for detection while transmitting rest of the light in thebroadcast-and-select optical WDM signals; a first node-to-node opticaltransmitter to produce the node-to-node optical WDM signal at a firstnode-to-node WDM wavelength; a first optical coupler to receive thenode-to-node optical WDM signal from the first node-to-node opticaltransmitter and to split the node-to-node optical WDM signal into afirst part of the node-to-node optical WDM signal and a second part ofthe node-to-node optical WDM signal; a second node-to-node opticaltransmitter to produce a second node-to-node optical WDM signal at asecond node-to-node WDM wavelength different from the first node-to-nodeWDM wavelength; a second optical coupler to receive the secondnode-to-node optical WDM signal from the second node-to-node opticaltransmitter and to split the second node-to-node optical WDM signal intoa first part of the second node-to-node optical WDM signal and a secondpart of the second node-to-node optical WDM signal; a first optical addmodule to combine the first part of the node-to-node optical WDM signaland the first part of the second node-to-node optical WDM signal into afirst add signal; a second optical add module to combine the second partof the node-to-node optical WDM signal and the second part of the secondnode-to-node optical WDM signal into a second add signal; a firstoptical add coupler coupled to the first fiber to direct the first addsignal to the first fiber; a second optical add coupler coupled to thesecond fiber to direct the second add signal to the second fiber; afirst optical drop coupler coupled to the first fiber upstream from thefirst optical add coupler to selectively couple light of thenode-to-node optical WDM signal and the second node-to-node optical WDMsignal out of the first fiber while transmitting light of thebroadcast-and-select optical WDM signals; a first optical drop module toseparate the light of the node-to-node optical WDM signal and the secondnode-to-node optical WDM signal from the first optical drop coupler intoa first drop part of the node-to-node optical WDM signal and a firstdrop part of the second node-to-node optical WDM signal; a secondoptical drop coupler coupled to the second fiber upstream from thesecond optical add coupler to selectively couple light of thenode-to-node optical WDM signal and the second node-to-node optical WDMsignal out of the second fiber while transmitting light of thebroadcast-and-select optical WDM signals; a second optical drop moduleto separate the light of the node-to-node optical WDM signal and thesecond node-to-node optical WDM signal from the second optical dropcoupler into a second drop part of the node-to-node optical WDM signaland a second drop part of the second node-to-node optical WDM signal;and a first optical receiver to receive the first and second drop partsof the node-to-node optical WDM signal; and a second optical receiver toreceive the first and second drop parts of the second node-to-nodeoptical WDM signal.
 11. The system as in claim 10, wherein each nodecomprises an electronic switching protection mechanism to provideswitching for the node-to-node communication traffic.
 12. The system asin claim 1, wherein: the optical ring nodes comprise reconfigurableoptical add and drop multiplexers each operable to add or drop one ormore of optical WDM channels; and the node-to-node optical WDM signal isat a WDM wavelength that is different from WDM wavelengths of theoptical WDM channels of the reconfigurable optical add and dropmultiplexers.
 13. An optical communication system, comprising: aplurality of optical ring nodes connected to form an optical ring havinga first optical ring path to carry light through the optical ring nodesalong a first direction and a second optical ring path to carry lightthrough the optical ring nodes along a second, opposite direction,wherein the optical ring is configured to support (1)broadcast-and-select optical WDM signals carrying communication trafficto the optical ring nodes without regeneration at each optical ring nodein each of the first and second optical ring paths, and (2) at least onenode-to-node optical WDM signal carrying communication traffic from oneoptical ring node to an adjacent optical ring node by regeneration ateach optical ring node in each of the first and second optical ringpaths, and wherein each ring node comprises: a broadcast-and-select addand drop module to produce at least one of the broadcast-and-selectoptical WDM signals and to split a fraction of light in thebroadcast-and-select optical WDM signals for detection whiletransmitting rest of the light in the broadcast-and-select optical WDMsignals; a node-to-node optical transmitter to produce the node-to-nodeoptical WDM signal; an optical coupler to receive the node-to-nodeoptical WDM signal from the node-to-node optical transmitter and tosplit the node-to-node optical WDM signal into a first node-to-nodeoptical WDM signal and a second node-to-node optical WDM signal; a firstoptical add coupler coupled to the first optical ring path to direct thefirst node-to-node optical WDM signal to the first optical ring path; asecond optical add coupler coupled to the second optical ring path todirect the second node-to-node optical WDM signal to the second opticalring path; a first optical drop coupler coupled to the first opticalring path upstream from the first optical add coupler to selectivelycouple light of the first node-to-node optical WDM signal out of thefirst optical ring path while transmitting light of thebroadcast-and-select optical WDM signals; a second optical drop couplercoupled to the second optical ring path upstream from the second opticaladd coupler to selectively couple light of the second node-to-nodeoptical WDM signal out of the second optical ring path whiletransmitting light of the broadcast-and-select optical WDM signals; andan optical receiver coupled to receive the first and second node-to-nodeoptical WDM signals from the first and second optical drop couplers. 14.The system as in claim 15, comprising: an optical probe transmittercoupled to the optical ring network to supply to the optical ringnetwork an optical probe signal at a probe wavelength which is differentfrom wavelengths of the broadcast-and-select optical WDM signals and thenode-to-node optical WDM signal; a probe monitor in at least one opticalring node to split a portion of the optical probe signal and to monitorthe optical probe signal to detect an optical failure in the opticalring network; and a protection switch in the optical ring network tocreate a default optical break point when the probe monitor indicatesthat there is no optical failure in the optical ring network and toclose the default optical break point when the probe monitor indicatesthat there is an optical failure, wherein the optical ring network isresponsive to a status of the monitored optical probe signal to controlthe protection switch and the optical ring nodes to sustaincommunications in the optical ring network outside a location of theoptical failure and to restore communications in the optical ringnetwork after the optical failure is repaired.
 15. The system as inclaim 14, comprising: an optical supervision channel (OSC) mechanismoperable to produce an OSC signal at an OSC wavelength between twoadjacent two optical ring nodes in each of the first and second fibersto communicate information on the ring network other than data traffic,and wherein the optical ring network is responsive to a status of theOSC signal, in addition to the status of the monitored optical probesignal, to control the protection switch and the optical ring nodes tosustain communications in the optical ring network outside a location ofthe optical failure and to restore communications in the optical ringnetwork after the optical failure is repaired.
 16. The system as inclaim 13, wherein: the optical ring nodes comprise reconfigurableoptical add and drop multiplexers each operable to add or drop one ormore of optical WDM channels; and the node-to-node optical WDM signal isat a WDM wavelength that is different from WDM wavelengths of theoptical WDM channels of the reconfigurable optical add and dropmultiplexers.
 17. A method for optical communication in a ring network,comprising: using broadcast-and-select optical WDM signals at differentwavelengths to carry communication traffic to optical ring nodes in thering network, each optical ring node splitting a fraction of thebroadcast-and-select optical WDM signals for detection whiletransmitting rest of light of the broadcast-and-select optical WDMsignals to a next optical ring node; using at least one node-to-nodeoptical WDM signal carrying node-to-node communication traffic from oneoptical ring node to an adjacent optical ring node by regeneration ateach optical ring node, wherein the node-to-node optical WDM signal isat a wavelength different from wavelengths of the broadcast-and-selectoptical WDM signals; designating each broadcast-and-select optical WDMsignal to a selected optical ring node for sending data from theselected optical ring node to one or more other optical ring nodes; andoperating the optical ring nodes to share a bandwidth of thenode-to-node optical WDM signal to transmit data from one optical ringnode to one or more other optical ring nodes through the node-to-nodecommunication traffic.
 18. The method as in claim 17, comprising:selecting an optical ring node that has a node traffic less than athreshold value to transmit data to one or more other optical ring nodesthrough the shared bandwidth of the node-to-node optical WDM signal onlywithout designating a broadcast-and-select optical WDM signal to theselected optical ring node.
 19. The method as in claim 17, comprising:selecting an optical ring node that has a node traffic higher than athreshold value to designate a broadcast-and-select optical WDM signalto the selected optical ring node for transmitting data to one or moreother optical ring nodes.
 20. The method as in claim 18, comprising:operating the selected optical ring node to use, in addition to thedesignated broadcast-and-select optical WDM signal, the shared bandwidthof the node-to-node optical WDM signal to transmit data to one or moreother optical ring nodes.
 21. The method as in claim 17, comprising:selecting an optical ring node, based on a level of quality of serviceor a level of communication latency associated with the selected opticalring node, to designate a broadcast-and-select optical WDM signal to theselected optical ring node for transmitting data to one or more otheroptical ring nodes.
 22. The method as in claim 21, comprising: operatingthe selected optical ring node to use, in addition to the designatedbroadcast-and-select optical WDM signal, the shared bandwidth of thenode-to-node optical WDM signal to transmit data to one or more otheroptical ring nodes.
 23. The method as in claim 17, comprising: in theoptical ring network, circulating an optical probe signal at a probewavelength which is different from wavelengths of thebroadcast-and-select optical WDM signals and the node-to-node opticalWDM signal; in at least one optical ring node, splitting a portion ofthe optical probe signal to monitor the optical probe signal to detectan optical failure in the optical ring network; and operating aprotection switch in the optical ring network to create a defaultoptical break point when the optical probe signal indicates that thereis no optical failure in the optical ring network and to close thedefault optical break point when the optical probe signal indicates thatthere is an optical failure.
 24. The method as in claim 23, comprising:operating an optical supervision channel (OSC) mechanism to produce anOSC signal at an OSC wavelength between two adjacent two optical ringnodes to communicate information on the ring network other than datatraffic; and operating the optical ring network in response to a statusof the OSC signal, in addition to the status of the monitored opticalprobe signal, to control the protection switch and the optical ringnodes.
 25. An optical communication system, comprising: a plurality ofoptical ring nodes connected to form an optical ring which support (1)broadcast-and-select optical WDM signals carrying communication trafficto the optical ring nodes without regeneration at each optical ringnode, and (2) at least one node-to-node optical WDM signal carryingcommunication traffic from one optical ring node to an adjacent opticalring node by regeneration at each optical ring node, wherein thenode-to-node optical WDM signal is at a wavelength different fromwavelengths of the broadcast-and-select optical WDM signals; and acentral node connected to the optical ring and structured to comprise anoptical-to-electrical-to-optical (OEO) block which converts receivedbroadcast-and-select optical WDM signals into electrical signals and toregenerate the broadcast-and-select optical WDM signals that travel tothe optical ring nodes, the central node structured to support the onenode-to-node optical WDM signal carrying communication traffic from anadjacent optical ring node to another adjacent optical ring node byregeneration of the one node-to-node optical WDM signal.
 26. The systemas in claim 25, comprising: a plurality of optical amplifiers coupled inthe optical ring and operable to amplify light in a gain spectral rangecovering optical wavelengths of the broadcast-and-select optical WDMsignals.
 27. The system as in claim 25, wherein: the optical ringincludes first and second fibers to connect two adjacent nodes to carryoptical traffic in opposite directions, respectively, and the systemcomprising: an optical supervision channel (OSC) mechanism operable toproduce an OSC signal at an OSC wavelength between two adjacent twooptical ring nodes in each of the first and second fibers to communicateinformation on the ring network other than data traffic, the OSCmechanism comprising: in each optical ring node and the central node, afirst OSC optical module that is coupled to the first and second fibersdownstream in the first fiber from the optical add and drop module andcomprises: a first OSC optical transmitter to produce the OSC signal forthe first fiber to a downstream optical node in the first fiber; a firstOSC add coupler coupled to the first fiber to direct the OSC signal intothe first fiber towards the downstream optical node in the first fiber;a first OSC drop coupler coupled to the second fiber upstream to theoptical add and drop module to selectively couple light of the OSCoptical WDM signal from an upstream optical node in the second fiber outof the second fiber while transmitting light of other wavelengths; and afirst OSC receiver coupled to receive the OSC signal from the first OSCdrop coupler; and a second OSC optical module that is coupled to thefirst and second fibers upstream in the first fiber to the optical addand drop module and comprises: a second OSC transmitter to produce theOSC signal for the second fiber to a downstream optical node in thesecond fiber; a second OSC add coupler coupled to the second fiber todirect the OSC signal into the second fiber towards the downstreamoptical node in the second fiber; a second OSC drop coupler coupled tothe first fiber upstream to the optical add and drop module toselectively couple light of the OSC signal from an upstream optical nodein the first fiber out of the first fiber while transmitting light ofother wavelengths; and a second OSC receiver coupled to receive the OSCsignal from the second optical drop coupler.
 28. The system as in claim27, wherein the central node is responsive to a status of the OSC signalto control a protection switching mechanism within the OEO block tosustain communications in the optical ring network when the OSC signalindicates an optical failure.
 29. The system as in claim 25, wherein:each optical ring node comprises a reconfigurable optical add and dropmultiplexer; and the node-to-node optical WDM signal is at a WDMwavelength that is different from WDM wavelengths of the optical WDMchannels of the reconfigurable optical add and drop multiplexers.